Among the very first language keywords one comes across when learning VBA, is the Dim keyword; declaring and using variables is easily the first step one takes on their journey away from the macro recorder.
About Scopes
Before we can really understand what variables do and what they’re useful for, we need to have a minimal grasp of the concept of scoping. When you record a macro, the executable instructions for you inside a procedure scope that’s delimited with Sub and End Sub tokens (tokens are the grammatical elements of the language, not necessarily single keywords), with the identifier name of the macro after the Sub keyword:
Sub DoSomething()
' executable code goes here
End Sub
Exactly none of the above code is executable, but compiling it creates an entry point that the VBA runtime can invoke and execute, because the procedure is implicitly public and as such, can be accessed from outside the “Module1” module it exists in (with or without Option Private Module). In other words the above code could tell us explicitly what the scope of the DoSomething procedure is, using the Public keyword before the Sub token:
Public Sub DoSomething()
' executable code goes here
End Sub
If we used Private instead, then Excel (or whatever the host application is) could not “see” it, so you would no longer find DoSomething in the list of available macros, and other modules in the same VBA project couldn’t “see” or invoke it either; a private procedure is only callable from other procedures in the same module.
Standard modules are themselves public, so you can refer to them from any other module in your project, and invoke their public members using the member access operator, the dot:
Public Sub DoStuff()
Module1.DoSomething
End Sub
Because public members of public modules become part of a global namespace, the public members can be referred to without an explicit qualifier:
Public Sub DoStuff()
DoSomething
End Sub
While convenient to type, it also somewhat obscures exactly what code is being invoked: without an IDE and a “navigate to definition” command, it would be pretty hard to know where that other procedure is located.
The global namespace contains not only the public identifiers from your VBA project, but also all the public identifiers from every referenced library, and they don’t need to be qualified either so that’s how you can invoke the VBA.Interaction.MsgBox function without qualifying with the library or module it’s defined in. If you write your own MsgBox function, every unqualified MsgBox call in that project is now invoking that new custom function, because VBA always prioritizes the host VBA project’s own type library over the referenced ones (every VBA project references the VBA standard library and the type library that defines the COM extension and automation model for the host application).
But that’s all going outward from a module: within a module, there are two levels of scoping: module level members can be accessed from anywhere in the module, and procedure level declarations can be accessed from anywhere inside that procedure.
Module-level declarations use Public and Private modifiers, and procedure-level ones use the Dim keyword. Dim is legal at module level too, but because Private and Public are only legal at module level (you can’t use them for procedure scope / “local” declarations), Rubberduck encourages you to use Dim for locals only.
For example a variable declared in a conditional block is allocated regardless of the state when the condition gets evaluated, and a variable declared inside a loop body is the same variable outside that loop, and for every iteration of that loop as well.
Non-Executable Statements
Procedures don’t only contain executable instructions: Dim statements, like statements with Private and Public modifiers, are declarative and do not do anything. You cannot place a debugger breakpoint (F9) on such statements, either. This is important to keep in mind: the smallest scope in VBA is the procedure scope, and it includes the parameters and all the local declarations of that procedure – regardless of where in the procedure body they’re declared at, so the reason to declare variables as you need them has more to do with reducing mental load and making it easier to extract a method by moving a chunk of code into another procedure scope. Declaring all locals at the top of a procedure often results in unused variables dangling, because of the constant up-and-down, back-and-forth scrolling that inevitably happens when a procedure eventually grows.
Const statements (to declare constant values) are also legal in local/procedure scope, and they’re identically non-executable; the same applies to Static declarations (variables that retain their value between invocations).
ReDim statements however are executable, even though they also count as a compile-time declaration – but they don’t count as a duplicate declaration, so the presence of ReDim doesn’t really justify skipping an initial Dim declaration.
Explicitness as an Option
Not only access modifiers can be implicit in VBA; the language lets you define a Variant variable on the fly, without a prior explicit declaration. If this behavior is practical for getting the job done and will indeed work perfectly fine, it’s also unnecessarily putting you at risk of typos that will only become a problem at run-time, if you’re lucky close enough to the source of the problem to hunt down and debug. By specifying Option Explicit at the top of every module, the compiler will treat implicit declarations as compile-time errors, telling you about the problem before it even becomes one.
Option Explicit has its limits though, and won’t protect you from typos in late-bound member calls, where invoking a member that doesn’t exist on a given object throws error 438 at run-time.
When to Declare a Variable
There are many reasons to declare a variable, but if you’re cleaning up macro recorder code the first thing you’ll want to do is to remove the dependency on Selection and qualifyRange and Cells member calls with a proper Worksheet object.
For example before might look like this:
Sub Macro1
Range("A10") = 42
Sheet2.Activate
Range("B10") = 42
End Sub
And after might look like this:
Public Sub Macro1()
Dim Sheet As Worksheet
Set Sheet = ActiveSheet
Sheet.Range("A10") = 42
Sheet2.Activate
Sheet.Range("B10") = 42
End Sub
The two procedures do exactly the same thing, but only one of them is doing it reliably. If the Sheet2 worksheet is already active, then there’s no difference and both versions produce identical output. Otherwise, one of them writes to whatever the ActiveSheet is, activates Sheet2, and then writes to that sheet.
There’s a notion of state in the first snippet that adds to the number of things you need to track and think about in order to understand what’s going on. Using variables, exactly what sheet is active at any point during execution has no impact whatsoever on the second snippet, beyond the initial assignment.
It’s that (global) state that’s behind erratic behavior such as code working differently when you leave it alone than when you step through – especially when loops start getting involved. Managing that global state makes everything harder than necessary.
Keep your state close, and your ducky closer, they say.
Set: With or Without?
Not being explicit can make the code read ambiguously, especially when you consider that objects in VBA can have default members. In the above snippets, the value 42 reads like it’s assigned to… the object that’s returned by the Range property getter of the Worksheet class. And that’s weird, because normally you would assign to a property of an object, not the object itself. VBA understands what it needs to do here, because the Range class says “I have a default member!” and that default member is implemented in such a way that giving it the value 42 does exactly the same as if the Range.Value member was being invoked explicitly. Because that behavior is an implementation detail, it means the only way to know is to read its documentation.
The Set keyword modifies an assignment instruction and says “we’re assigning an object reference”, so VBA doesn’t try to check if there’s a default member on the left-hand side of the assignment operator, and the compiler expects an object reference on the right-hand side, …and then only throws at run-time when that isn’t the case – but because this information is all statically available at compile-time, Rubberduck can warn about such suspicious assignments.
So to assign a variable that holds a reference to a Range object, we must use the Set keyword. To assign a variable that holds the value of a Range object, we must not use the Set keyword. Declaring an explicit data type for every variable (meaning not only declaring things, but also typing them) helps prevent very preventable bugs and subtle issues that can be hard to debug.
As SomethingExplicit
Whether Public or Private, whether local or global, most variables are better off with a specific data type using an As clause:
Dim IsSomething
Dim SomeNumber As Long
Dim SomeAmount As Currency
Dim SomeValue As Double
Dim SomeDateTime As Date
Dim SomeText As String
Dim SomeSheet As Worksheets
Dim SomeCell As Range
Using an explicit data/class/interface type, especially with objects, helps keep things early-bound, meaning both the compiler and static code analysis tools (like Rubberduck) can better tell what’s going on before the code actually gets to run.
We can often chain member calls; the Worksheets collection’s indexer necessarily yields a Worksheet object, no?
Public Sub Macro1()
ActiveWorkbook.Worksheets("Sheet1").Range("A1").Value = 42
End Sub
If you manually type this instruction, you’ll notice something awkward that should be unexpected when you type the dot operator after Worksheets(“Sheet1”), because the property returns an Object interface… which tells VBA it has members that can be invoked, but leaves no compile-time clue about any of them. That’s why the Range member call is late-bound and only resolved at run-time, and because the compiler has no idea what the members are until the code is running, it cannot populate the completion list with the members of Worksheet, and will merrily compile and attempt to invoke a Range member.
By breaking the chain and declaring variables, we restore compile-time validations:
Public Sub Macro1()
Dim Sheet As Worksheet
Set Sheet = ActiveWorkbook.Worksheets("Sheet2")
Sheet.Range("A1").Value = 42
End Sub
When NOT to Declare Variables
Variables are so nice, sometimes we declare them even when we don’t need them. There are many valid reasons to use a variable, including abstracting the result of an expression behind its value. Assuming every variable is assigned and referenced somewhere, there are still certain variables that are always redundant!
Objects are sneaky little things… not only can they have a default member that gets implicitly invoked, they can also have a default instance that lives in the global scope and is always named after the class it’s an instance of.
Declaring a local variable to hold a copy of a reference to an object that’s already globally accessible, is always redundant! Document modules (in Excel that’s ThisWorkbook and the Worksheet modules) and UserForms always have such a default instance:
Public Sub Macro1()
Dim WB As Workbook
Set WB = ThisWorkbook 'redundant and obscures intent!
Dim Sheet As Worksheet
Set Sheet = Sheet1 'redundant, just use Sheet1 directly!
End Sub
Sprinkle Generously
Variables are a simple but powerful tool in your arsenal. Using them enhances the abstraction level of your code, practices your brain to stop and think about naming things, can help prevent binding errors and remove implicit late-binding / keep your code entirely visible to the compiler and Rubberduck. Used wisely, variables can make a huge difference between messy and redundant macro-recorder code and squeaky-clean, professionally-written VBA code.
A little while ago already, I went and explored dynamic UI with MSForms in VBA through a lens tinted with Windows Presentation Foundation (WPF) concepts, and ended up implementing a working prototype Model-View-ViewModel (MVVM) framework for VBA… across a hundred and some modules covering everything from property and command bindings to input and model validation. I’m still planning to build an actual COM library for it one day – for now I’m entirely focused on everything around Rubberduck3.
Although… the last month or so has actually been mostly about publishing the new website and setting up the Ko-fi shop: the new website is not without issues (search links are broken, for one), but the source code ownership has been transferred to the rubberduck-vba organization on GitHub and I’m satisfied enough with it to move on.
But then there’s operating the shop. When an order comes in, there’s a worksheet (duh!) with a Sales table where I enter the invoice line items sold using a Stock Keeping Unit (SKU) code that identifies each item sold; the Inventory table picks up the sale and calculates a new Available to Sell figure.
But tracking items sold isn’t the whole picture: an Invoice table tracks the actual totals including the shipping charges and actual shipping costs (currently 24% underwater, but I’ve since adjusted the shipping charges to better reflect reality), computing the Cost of Goods Sold, and ultimately a profit margin.
So for each invoice, I know I need:
Invoice number and date
Billing/shipping information (name, address, etc.)
The number of units sold per SKU, with the amount paid by the customer
The shipping charge paid by the customer
And then I manually prepare the invoice document. Such a waste of time, right? Of course I couldn’t leave it at that – all I needed was a UserForm to enter all that, and a command that would update the merchandise planning workbook and prepare the invoice document for me.
Thing is, I wanted that form to use property bindings and some extent of MVVM, but I wasn’t going to import the 100+ modules of the old MVVM prototype code. So instead, I made a “lite” version.
Bindings and the propagation of property value changes are the core mechanics that make MVVM work, and we don’t need dozens of classes for that.
We do need INotifyPropertyChanged and IHandlePropertyChanged interfaces:
Option Explicit
Public Sub OnPropertyChanged(ByVal Source As Object, ByVal Name As String)
End Sub
Public Sub RegisterHandler(ByVal Handler As IHandlePropertyChanged)
End Sub
Option Explicit
Public Sub OnPropertyChanged(ByVal Source As Object, ByVal Name As String)
End Sub
These interfaces are important, because the bindings need to handle property changed events; the View Model needs to invoke the registered callbacks. This is used in place of actual events, because interfaces in VBA don’t expose events, and we want an abstraction around property changes, so that everything that needs to notify about property changes can do so in a standardized way.
The IHandlePropertyChanged interface is to be implemented by property binding classes, such as this TextBoxValueBinding class:
Option Explicit
Implements IHandlePropertyChanged
Private WithEvents UI As MSForms.TextBox
Private Type TBinding
Source As Object
SourceProperty As String
End Type
Private This As TBinding
Public Sub Initialize(ByVal Control As MSForms.TextBox, ByVal Source As Object, ByVal SourceProperty As String)
Set UI = Control
Set This.Source = Source
This.SourceProperty = SourceProperty
If TypeOf Source Is INotifyPropertyChanged Then RegisterPropertyChanges Source
End Sub
Private Sub RegisterPropertyChanges(ByVal Source As INotifyPropertyChanged)
Source.RegisterHandler Me
End Sub
Private Sub IHandlePropertyChanged_OnPropertyChanged(ByVal Source As Object, ByVal Name As String)
If Source Is This.Source And Name = This.SourceProperty Then
UI.Text = VBA.Interaction.CallByName(This.Source, This.SourceProperty, VbGet)
End If
End Sub
Private Sub UI_Change()
VBA.Interaction.CallByName This.Source, This.SourceProperty, VbLet, UI.Value
End Sub
A binding has a source and a target object and property; the source is a ViewModel object, and the target is a MSForms control, in this case a TextBox. The binding must handle the control’s events to update the source whenever the value of the target changes. In this limited version we’re only going to handle the Change event, but if we wanted we could go further and handle KeyDown here to implement input validation. Some error handling wouldn’t hurt, either.
Because everything that involves notifying about property changes is standardized through interfaces, we can make a PropertyChangeNotification helper class to register the handlers:
Option Explicit
Private Handlers As VBA.Collection
Public Sub AddHandler(ByVal Handler As IHandlePropertyChanged)
Handlers.Add Handler
End Sub
Public Sub Notify(ByVal Source As Object, ByVal Name As String)
Dim Handler As IHandlePropertyChanged
For Each Handler In Handlers
Handler.OnPropertyChanged Source, Name
Next
End Sub
Private Sub Class_Initialize()
Set Handlers = New VBA.Collection
End Sub
This class is responsible for holding a reference to a collection of handlers, and a Notify method invokes the OnPropertyChange method on each registered handler.
ViewModel
The OrderHeaderModel class is the binding source, so it exposes a property representing the value of each field in the form. The Property Let procedures are all structured as follows:
If current encapsulated value is not equal to the new value…
Set the current value to the new value
Notify of a property change
ViewModel classses need to implement INotifyPropertyChange, and the implementation simply uses an instance of the helper class above to do its thing:
Option Explicit
Implements INotifyPropertyChanged
Private Notification As New PropertyChangeNotification
'...
Private Sub OnPropertyChanged(ByVal Name As String)
INotifyPropertyChanged_OnPropertyChanged Me, Name
End Sub
Private Sub INotifyPropertyChanged_OnPropertyChanged(ByVal Source As Object, ByVal Name As String)
Notification.Notify Source, Name
End Sub
Private Sub INotifyPropertyChanged_RegisterHandler(ByVal Handler As IHandlePropertyChanged)
Notification.AddHandler Handler
End Sub
The private OnPropertyChanged method further simplifies the notification by providing the Source argument, which needs to be an instance of the ViewModel, so that’s always Me. So the properties all look more or less like this:
Public Property Get OrderNumber() As Long
OrderNumber = This.OrderNumber
End Property
Public Property Let OrderNumber(ByVal Value As Long)
If This.OrderDate <> Value Then
This.OrderNumber = Value
OnPropertyChanged "OrderNumber"
End If
End Property
The ViewModel is inherently domain-specific, so for a form that collects information about an order we’re going to be looking at properties like OrderNumber, OrderDate, BillToName, ShipToAddress, etc.; in another application, a ViewModel could be a completely different thing – it all really depends on what the thing is meant to do. But no matter what the domain is, a ViewModel will be implementing INotifyPropertyChanged as shown above.
View
Implementing the View (the form’s code-behind module) boils down to setting up all the necessary bindings, and we do this using a PropertyBindings helper module:
Option Explicit
'@Description "Binds a MSForms.Control property to a source property"
Public Function BindProperty(ByVal Control As MSForms.Control, ByVal ControlProperty As String, ByVal SourceProperty As String, ByVal Source As Object, Optional ByVal InvertBoolean As Boolean = False) As OneWayPropertyBinding
Dim Binding As OneWayPropertyBinding
Set Binding = New OneWayPropertyBinding
Binding.Initialize Control, ControlProperty, Source, SourceProperty, InvertBoolean
Set BindProperty = Binding
End Function
'@Description "Binds the Text/Value of a MSForms.TextBox to a source property"
Public Function BindTextBox(ByVal Control As MSForms.TextBox, ByVal SourceProperty As String, ByVal Source As Object) As TextBoxValueBinding
Dim Binding As TextBoxValueBinding
Set Binding = New TextBoxValueBinding
Binding.Initialize Control, Source, SourceProperty
Set BindTextBox = Binding
End Function
'@Description "Binds the Text of a MSForms.ComboBox to a String source property"
Public Function BindComboBox(ByVal Control As MSForms.ComboBox, ByVal SourceProperty As String, ByVal Source As Object) As ComboBoxValueBinding
Dim Binding As ComboBoxValueBinding
Set Binding = New ComboBoxValueBinding
Binding.Initialize Control, Source, SourceProperty
Set BindComboBox = Binding
End Function
'@Description "Binds the Value of a MSForms.CheckBox to a Boolean source property"
Public Function BindCheckBox(ByVal Control As MSForms.CheckBox, ByVal SourceProperty As String, ByVal Source As Object) As CheckBoxValueBinding
Dim Binding As CheckBoxValueBinding
Set Binding = New CheckBoxValueBinding
Binding.Initialize Control, Source, SourceProperty
Set BindCheckBox = Binding
End Function
As you can see each MSForms control gets its Binding class, and a OneWayPropertyBinding binds a source property to a target property without notifying for target changes (so without listening for control events) – this is useful for binding labels, ListBox/ComboBox contents, and anything else that doesn’t involve control events.
The form has a private ConfigureBindings method (invoked from the UserForm_Initialize handler) where we essentially map each one of the form controls to corresponding ViewModel properties:
This rather straightforward configuration completely replaces event handlers. That’s right: the bindings take care of the control events for us, so checking the ShipToSameBox checkbox automatically disables the ShipToNameLabel, ShipToAddressLabel, ShipToAddressLine1, ShipToAddressLine2, and ShipToAddressLine3 controls on the form, and un-checking it automatically enables them, and we don’t need to explicitly handle any control events to achieve this. Small note: here the View is accessing a table in InventorySheet directly, and it shouldn’t be doing that, because what SKUs are available belongs in the Model, not the View: I should instead implement a service that accesses the worksheet for me and supplies the available SKU codes.
With the form controls effectively abstracted away by the ViewModel, we never need to directly interact with MSForms to affect the View, because the property bindings do this automatically for us. This means commands can affect just the ViewModel, and doing that will automatically keep the View in sync.
Commands
This lite version of MVVM doesn’t (yet?) have command bindings, but UI commands are still abstracted behind an ICommand interface. In my case I needed a command to add a new order line item, so I implemented it like this:
Option Explicit
Implements ICommand
Private Function ICommand_CanExecute(ByVal Parameter As Object) As Boolean
ICommand_CanExecute = TypeOf Parameter Is OrderHeaderModel
End Function
Private Sub ICommand_Execute(ByVal Parameter As Object)
If Not TypeOf Parameter Is OrderHeaderModel Then Err.Raise 5
Dim Model As OrderHeaderModel
Set Model = Parameter
Dim Item As OrderLineItemModel
Set Item = New OrderLineItemModel
Item.SKU = Model.NewLineItem.SKU
Item.Quantity = Model.NewLineItem.Quantity
Item.Price = Model.NewLineItem.Price
Model.AddLineItem Item
End Sub
This code is completely oblivious of any form or form controls: it only knows about the OrderHeaderModel and OrderLineItemModel classes, and what it needs to do with them. Why bother implementing this in a separate class, rather than in the form’s code-behind?
Without command bindings, we do need to handle command buttons’ Click event:
Private Sub AddLineItemButton_Click()
CmdAddLineItem.Execute OrderModel
End Sub
I don’t like having logic in event handlers, so this one-liner is perfect. Without a command class the View would need to have more code, code that isn’t directly related to the View itself, and then the commands’ dependencies would become the View‘s dependencies, and that would be wrong: if I made a “Save to Database” button, I’d want the ADODB stuff anywhere but in the form’s code-behind; command classes can have their own dependencies, so pulling commands into their own classes keeps the View cohesive and focused on its purpose.
I’m finding that MVVM works best with relatively complex forms such as this one, where some fields’ enabled state might depend on some checkbox control’s value, for example. There’s something oddly satisfying typing something in a textbox and seeing another (disabled!) textbox get updated with the same content, knowing zero event handling is going on in the form.
Viability
If the full-featured MVVM framework isn’t viable in VBA, a more lightweight version of the UI paradigm certainly is: this particular VBA project doesn’t have dozens of class modules, and yet still manages to leverage what makes Model-View-ViewModel such a compelling architecture.
One of the objectively coolest features in Rubberduck is the Fakes API. Code that pops a MsgBox for example, needs a way to work without actually popping that message box, otherwise that code cannot be unit tested… without somehow hijacking the MsgBox function. The Fakes API does exactly that: it hooks into the VBA runtime, intercepts specific internal function calls, and makes it return exactly what your test setup …set up.
This API can stop time, or Now can be told to return 1:59AM on first invocation, 1:00AM on the next, and then we can test and assert that some time-sensitive logic survives a daylight savings time toggle, or how Timer-dependent code behaves at midnight.
Let’s take a look at the members of the IFakesProvider interface.
Fakes Provider
Fakes for many of the internal VBA standard library functions exist since the initial release of the feature, although some providers wouldn’t always play nicely together – thanks to a recent pull request from @tommy9 these issues have been resolved, and a merry bunch of additional implementations are now available in pre-release builds:
Name
Description
Parameter names
MsgBox
Configures VBA.Interaction.MsgBox calls
Fakes.Params.MsgBox
InputBox
Configures VBA.Interaction.InputBox calls
Fakes.Params.InputBox
Beep
Configures VBA.Interaction.Beep calls
Environ
Configures VBA.Interaction.Environ calls
Fakes.Params.Environ
Timer
Configures VBA.DateTime.Timer calls
DoEvents
Configures VBA.Interaction.DoEvents calls
Shell
Configures VBA.Interaction.Shell calls
Fakes.Params.Shell
SendKeys
Configures VBA.Interaction.SendKeys calls
Fakes.Params.SendKeys
Kill
Configures VBA.FileSystem.Kill calls
Fakes.Params.Kill
MkDir
Configures VBA.FileSystem.MkDir calls
Fakes.Params.MkDir
RmDir
Configures VBA.FileSystem.RmDir calls
Fakes.Params.RmDir
ChDir
Configures VBA.FileSystem.ChDir calls
Fakes.Params.ChDir
ChDrive
Configures VBA.FileSystem.ChDrive calls
Fakes.Params.ChDrive
CurDir
Configures VBA.FileSystem.CurDir calls
Fakes.Params.CurDir
Now
Configures VBA.DateTime.Now calls
Time
Configures VBA.DateTime.Time calls
Date
Configures VBA.DateTime.Date calls
Rnd*
Configures VBA.Math.Rnd calls
Fakes.Params.Rnd
DeleteSetting*
Configures VBA.Interaction.DeleteSetting calls
Fakes.Params.DeleteSetting
SaveSetting*
Configures VBA.Interaction.SaveSetting calls
Fakes.Params.SaveSetting
Randomize*
Configures VBA.Math.Randomize calls
Fakes.Params.Randomize
GetAllSettings*
Configures VBA.Interaction.GetAllSettings calls
SetAttr*
Configures VBA.FileSystem.SetAttr calls
Fakes.Params.SetAttr
GetAttr*
Configures VBA.FileSystem.GetAttr calls
Fakes.Params.GetAttr
FileLen*
Configures VBA.FileSystem.FileLen calls
Fakes.Params.FileLen
FileDateTime*
Configures VBA.FileSystem.FileDateTime calls
Fakes.Params.FileDateTime
FreeFile*
Configures VBA.FileSystem.FreeFile calls
Fakes.Params.FreeFile
IMEStatus*
Configures VBA.Information.IMEStatus calls
Dir*
Configures VBA.FileSystem.Dir calls
Fakes.Params.Dir
FileCopy*
Configures VBA.FileSystem.FileCopy calls
Fakes.Params.FileCopy
*Members marked with an asterisk are only available in pre-release builds for now.
Parameter Names
The IVerify.ParameterXyz members make a unit test fail if the specified parameter wasn’t given a specified value, but the parameter names must be passed as strings. This is a UX issue: the API essentially requires hard-coded magic string literals in its users’ code; this is obviously error-prone and feels a bit arcane to use. The IFakesProvider interface has been given a Params property that gets an instance of a class that exposes the parameter names for each of the IFake implementations, as shown in the list above, and the screenshot below:
Picking the correct parameter name from a drop-down completion list beats risking a typo, doesn’t it?
Note: the PR for this feature has not yet been merged at the time of this writing.
Testing Without Fakes (aka Testing with Stubs)
Unit tests have a 3-part structure: first we arrange the test, then we act by invoking the method we want to test; lastly, we assert that an actual result matches the expectations. When using fakes, we configure them in the arrange part of the test, and in the assert part we can verify whether (and/or how many times) a particular method was invoked with a particular parameterization.
Let’s say we had a procedure we wanted to write some tests for:
Public Sub TestMe()
If MsgBox("Print random number?", vbYesNo + vbQuestion, "Test") = vbYes Then
Debug.Print Now & vbTab & Rnd * 42
Else
Debug.Print Now
End If
End Sub
If we wanted to make this logic fully testable without the Fakes API, we would need to inject (likely as parameters) abstractions for MsgBox, Now, and Debug dependencies: instead of invoking MsgBox directly, the procedure would be invoking the Prompt method of an interface/class that wraps the MsgBox functionality. Unit tests would need a stub implementation of that interface in order to allow some level of configuration setup – an invocation counter, for example. A fully testable version of the above code might then look like this:
Public Sub TestMe(ByVal MessageBox As IMsgBox, ByVal Random As IRnd, ByVal DateTime As IDateTime, ByVal Logger As ILogger)
If MessageBox.Prompt("Print random number?", "Test") = vbYes Then
Logger.LogDebug DateTime.Now & vbTab & Random.Next * 42
Else
Logger.LogDebug DateTime.Now
End If
End Sub
The method is testable, because the caller controls all the dependencies. We’re probably injecting an IMsgBox that pops a MsgBox, an IRnd that wraps Rnd, a DateTime parameter that returns VBA.DateTime.Now and an ILogger that writes to the debug pane, but we don’t know any of that. I fact, we could very well run this method with an ILogger that writes to some log file or even to a database; the IRnd implementation could consistently be returning 0.4 on every call, IDateTime.Now could return Now adjusted to UTC, and IMsgBox might actually display a fancy custom modal UserForm dialog – either way, TestMe doesn’t need to change for any of that to happen: it does what it needs to do, in this case fetching the next random number and outputting it along with the current date/time if a user prompt is answered with a “Yes”, otherwise just output the current date/time. It’s the interfaces that provide the abstraction that’s necessary to decouple the dependencies from the logic we want to test. We could implement these interfaces with stubs that simply count the number of times each member is invoked, and the logic we’re testing would still hold.
We could then write tests that validate the conditional logic:
'@TestMethod
Public Sub TestMe_WhenPromptYes_GetsNextRandomValue()
' Arrange
Dim MsgBoxStub As StubMsgBox ' implements IMsgBox, but we want the stub functionality here
Set MsgBoxStub = New StubMsgBox
MsgBoxStub.Returns vbYes
Dim RndStub As StubRnd ' implements IRnd, but we want the stub functionality here too
Set RndStub = New StubRnd
' Act
Module1.TestMe MsgBoxStub, RndStub, New DateTimeStub, New LoggerStub
' Assert
Assert.Equals 1, RndStub.InvokeCount
End Sub
'@TestMethod
Public Sub TestMe_WhenPromptNo_DoesNotGetNextRandomValue()
' Arrange
Dim MsgBoxStub As StubMsgBox
Set MsgBoxStub = New StubMsgBox
MsgBoxStub.Returns vbNo
Dim RndStub As StubRnd
Set RndStub = New StubRnd
' Act
Module1.TestMe MsgBoxStub, RndStub, New DateTimeStub, New LoggerStub
' Assert
Assert.Equals 0, RndStub.InvokeCount
End Sub
These stub implementations are class modules that need to be written to support such tests. StubMsgBox would implement IMsgBox and expose a public Returns method to configure its return value; StubRnd would implement IRnd and expose a public InvokeCount property that returns the number of times the IRnd.Next method was called. In other words, it’s quite a bit of boilerplate code that we’d usually rather not need to write.
Let’s see how using the Fakes API changes that.
Using Rubberduck.FakesProvider
The standard test module template defines Assert and Fakes private fields. When early-bound (needs a reference to the Rubberduck type library), the declarations and initialization look like this:
'@TestModule
Option Explicit
Option Private Module
Private Assert As Rubberduck.AssertClass
Private Fakes As Rubberduck.FakesProvider
'@ModuleInitialize
Public Sub ModuleInitialize()
Set Assert = CreateObject("Rubberduck.AssertClass")
Set Fakes = CreateObject("Rubberduck.FakesProvider")
End Sub
The Fakes API implements three of the four stubs for us, so we still need an implementation for ILogger, but now the method remains fully testable even with direct MsgBox, Now and Rnd calls:
Public Sub TestMe(ILogger Logger)
If MsgBox("Print random number?", vbYesNo + vbQuestion, "Test") = vbYes Then
Logger.LogDebug Now & vbTab & Rnd * 42
Else
Logger.LogDebug Now
End If
End Sub
With an ILogger stub we could write a test that validates what’s being logged in each conditional branch (or we could decide that we don’t need an ILogger interface and we’re fine with tests actually writing to the debug pane, and leave Debug.Print statements in place), but let’s just stick with the same two tests we wrote above without the Fakes API. They look like this now:
'@TestMethod
Public Sub TestMe_WhenPromptYes_GetsNextRandomValue()
' Arrange
Fakes.MsgBox.Returns vbYes
' Act
Module1.TestMe New LoggerStub ' ILogger is irrelevant for this test
' Assert
Fakes.Rnd.Verify.Once
End Sub
'@TestMethod
Public Sub TestMe_WhenPromptNo_DoesNotGetNextRandomValue()
' Arrange
Fakes.MsgBox.Returns vbNo
' Act
Module1.TestMe New LoggerStub ' ILogger is irrelevant for this test
' Assert
Fakes.Rnd.Verify.Never
End Sub
We configure the MsgBox fake to return the value we need, we invoke the method under test, and then we verify that the Rnd fake was invoked once or never, depending on what we’re testing. A failed verification will fail the test the same as a failed Assert call.
The fakes automatically track invocations, and remember what parameter values each invocation was made with. Setup can optionally supply an invocation number (1-based) to configure specific invocations, and verification can be made against specific invocation numbers as well, and we could have a failing test that validates whether Randomize is invoked when Rnd is called.
API Details
The IFake interface exposes members for the setup/configuration of fakes:
Name
Description
AssignsByRef
Configures the fake such as an invocation assigns the specified value to the specified ByRef argument.
Passthrough
Gets/sets whether invocations should pass through to the native call.
RaisesError
Configures the fake such as an invocation raises the specified run-time error.
Returns
Configures the fake such as the specified invocation returns the specified value.
ReturnsWhen
Configures the fake such as the specified invocation returns the specified value given a specific parameter value.
Verify
Gets an interface for verifying invocations performed during the test. See IVerify.
The members of Rubberduck.IFake
The IVerify interface exposes members for verifying what happened during the “Act” phase of the test:
Name
Description
AtLeast
Verifies that the faked procedure was called a specified minimum number of times.
AtLeastOnce
Verifies that the faked procedure was called one or more times.
AtMost
Verifies that the faked procedure was called a specified maximum number of times.
AtMostOnce
Verifies that the faked procedure was not called or was only called once.
Between
Verifies that the number of times the faked procedure was called falls within the supplied range.
Exactly
Verifies that the faked procedure was called a specified number of times.
Never
Verifies that the faked procedure was called exactly 0 times.
Once
Verifies that the faked procedure was called exactly one time.
Parameter
Verifies that the value of a given parameter to the faked procedure matches a specific value.
ParameterInRange
Verifies that the value of a given parameter to the faked procedure falls within a specified range.
ParameterIsPassed
Verifies that an optional parameter was passed to the faked procedure. The value is not evaluated.
ParameterIsType
Verifies that the passed value of a given parameter was of a type that matches the given type name.
The members of Rubberduck.IVerify
There’s also an IStub interface: it’s a subset of IFake, without the Returns setup methods. Thus, IStub is used for faking Sub procedures, and IFake for Function and Property procedures.
When to Stub Standard Library Members
Members of VBA.FileSystem not covered include EOF and LOF functions, Loc, Seek, and Reset. VBA I/O keywords Name, Open, and Close operate at a lower level than the standard library and aren’t covered, either. VBA.Interaction.CreateObject and VBA.Interaction.GetObject, VBA.Interaction.AppActivate, VBA.Interaction.CallByName, and the hidden VBA.Interaction.MacScript function, aren’t implemented.
Perhaps CreateObject and GetObject calls belong behind an abstract factory and a provider interface, respectively, and perhaps CallByName doesn’t really need hooking anyway. In any case there are a number of file I/O operations that cannot be faked and demand an abstraction layer between the I/O work and the code that commands it: that’s when you’re going to want to write stub implementations.
If you’re writing a macro that makes an HTTP request and processes its response, consider abstracting the HttpClient stuff behind an interface (something like Function HttpGet(ByVal Url As String)): the macro code will gain in readability and focus, and then if you inject that interface as a parameter, then a unit test can inject a stub implementation for it, and you can write tests that handle (or not?) an HTTP client error, or process such or such JSON or HTML payload – without hitting any actual network and making any actual HTTP requests.
Until we can do mocking with Rubberduck, writing test stubs for our system-boundary interfaces is going to have to be it. Mocking would remove the need to explicitly implement most test stubs, by enabling the same kind of customization as with fakes, but with your own interfaces/classes. Or Excel’s. Or anything, in theory.
The next major version of Rubberduck is currently in [very] early development stages – saying that there is a lot of work ahead would be quite an understatement, but the skeleton is slowly taking shape, and things are looking very, very good.
Since the beginning of the project, Rubberduck’s user interface components (other than dialogs) have always been hosted in traditional, native dockable toolwindows. We built everything on top of the VBIDE editor, using Office CommandBar UI to simulate a status bar and make up for the lack of in-editor integration. Over the years this early design decision slowly became a burden: tearing down the many dockable toolwindows contributed to a pesky access violation crash on exit, low-level hooks for keyboard shortcuts constantly need to detach and re-attach as focus switches between the VBE main window and other applications, autocompletion/self-closing pairs was a nightmare to implement, and while the all-or-nothing approach to parsing made it so that we could always assume we were looking at valid VBA code that could be compiled, it also painted us into a corner where actually moving towards what we wanted Rubberduck to achieve by v3.0 would be extremely difficult, if not impossible.
Behold, the Rubberduck Editor
Rubberduck’s input was always driven by the Visual Basic Editor – now the code in the VBE is going to be output by Rubberduck. Of course, the code will go both ways, but now hidden attributes probably won’t need to be hidden anymore, and the editor can now be exactly what we envision it to be.
There will only be a single toolwindow that will host the editor and UI components like the Code Explorer. At this early stage my focus is entirely on the editor itself, but the idea is ultimately to get actual document tabs and a more practical and friendly docking manager.
Here’s what it looks like as of this writing:
The dropdowns don’t have a real item source yet, but the mock data gives a good idea of what it’s going to be like to edit VBA code with Rubberduck in the future.
Typing “Sub” and hitting the spacebar immediately completes the block and places a new folding node:
The faint dotted underline under “Sub” is a text marker; the editor has the ability to display various such markers at the exact desired position in the document, so we will be using them to show inspection results right there – with tooltips:
Hint-level results will be denoted with this dotted underline indicator; suggestion level will be a green squiggly underline, warnings a blue squiggle, and error level results will appear as red squiggles:
There will also be a new “ducky button” that pops up when the caret is on one such marker, and lets you pick a quick-fix in-place to address an inspection result:
The indenter still needs to be wired up, but this editor will ultimately indent your code as you type it. All the autocompletion features also need to be ported over to work here, and then we’ll want searchable and filterable IntelliSense, parameter info tooltips, and we’ll need to simulate the VBIDE “prettification” that occurs when a line is validated, so that public sub becomes Public Sub and identifiers take the casing they’re declared with.
We get an undo stack that can handle much more than 20 steps, and did I mention the status bar?
For now, all it does is report the current caret position in the editor, but Rubberduck 3.0 will be using it to report parsing progress, instead of the CommandBar button/label we’ve been abusing forever.
There will probably still be a command bar of some sort, but it will be part of the WPF/XAML managed UI; the old Rubberduck CommandBar will be decommissioned.
The one thing that’s 100% guaranteed to not happen in the new Rubberduck editor, is everything that needs to happen beyond design-time: there is no hook into the VBIDE debugger, so Rubberduck has no way of tracking the current instruction. As a result, the editor will be sadly useless in debug mode.
The editor work is just the beginning: Rubberduck 3.0 currently doesn’t even have a parser, let alone any inspections. In the next few months, the very heart of Rubberduck will be reworked to function with the new editor. It’s essentially like rewriting Rubberduck, but with an editor we fully control instead of one we constantly need to fight with.
Meanwhile v2.5.2 is approaching 25K downloads, and there’s quite a bit of work in 2.5.x that hasn’t been “officially” released yet, including everything that happened during a very successful Hacktoberfest 2022: we’ll be releasing v2.5.3 in the near future – stay tuned!
As Rubberduck started to beef up its static code analysis capabilities in late 2015, it became evident that writing VBA (or VB6) code with Rubberduck loaded up in the Visual Basic Editor (VBE) would inevitably change not only how we work in VBA, but also how we write our VBA code in the first place.
“Rubberduck is essentially providing a bridge between VBA land where people just get in and have a go and the VS land where if you don’t know a great deal about software development, you just waste your time and burn. Rubberduck will put a lot of people on a big learning curve and this will result in a lot of questions.” – AndrewM-commented on Oct 9, 2015
There’s an old issue (#823, still opened as of this writing) about having a coding style guide somewhere, that would enshrine the philosophy behind what Rubberduck is, in a way, trying to make your code-writing be/become; I think that was a great idea and I’m hoping this post captures the essence of it, at least as far as thinking code goes.
About Code Inspections
If you fire up Rubberduck on any legacy VBA project with any significant amount of code, there’s a very high probability that static code analysis generates tons of inspection results, for various mundane little things. Should your goal be to quick-fix all the things and have code that doesn’t spawn any Rubberduck inspection results?
Perhaps surprisingly, the answer is a resounding “no”.
Severity Levels
In Rubberduck each inspection has a configurable “severity level” that defaults to Warning for most inspections (it’s the default-unless-specified-otherwise for all Rubberduck inspections):
Error level indicates a potential problem you likely want to pay immediate attention to, because it could be (or cause) a bug. If inspection results rendered in the code pane, these would be red squiggly underlines.
Warning level indicates a potential issue you should be aware of.
Suggestion level is usually used for various opportunities, not necessarily problems.
Hint level is also for various non-problematic opportunities. If inspection results rendered in the code pane, these would be a subtle dotted underline with a hover text.
DoNotShow disables the inspection: not only its results won’t show, they won’t even be generated.
By default, Rubberduck is configured to run all (that’s currently over 110, counting the hidden/Easter egg ones) inspections, with a handful of cherry-picked exceptions for inspections that would be flagging the exact opposite situation that another enabled inspection is already flagging – for example we ship implicit ByRef modifier enabled (as a Hint), but redundant ByRef modifier is disabled unless you give it a severity level that’s anything other than DoNotShow. This avoids “fixing” one inspection result only to get a new one flagging the exact opposite, which would be understandably confusing for users that aren’t familiar with static code analysis tooling.
Are inspections somehow imbued with the knowledge of whether you should treat them as errors, warnings, or mere hints and suggestions? Sometimes, yes. Missing Option Explicit should make a clear consensus at Error level. On the flipside, whether an implicit default member call or the use of an empty string literal should be a Warning, a Hint, or shown at all probably depends more on how comfortable or experienced you are with VBA/VB6 as a language, or could be just a personal preference; what matters is that the static code analysis tooling is letting you know something about the code, that the code alone isn’t necessarily saying.
Philosophy
One of the very first inspection to be implemented in Rubberduck was the Option Explicit inspection. Okay, part of it was just because it was a trivial one to implement even before we had an actual parser… but the basic idea was (and still is) that nobody knows everything, and it’s with our combined knowledge that we make a mighty bunch, and that is why static code analysis in Rubberduck explains the reasoning behind each inspection result: there are quite many things Rubberduck warns of, that I had no idea about 10 or 15 years ago. That never stopped me (and won’t stop you either) from writing VBA code that worked perfectly fine (except when it didn’t), but whether we realize and accept it or not… a macro written in VBA code is a set of executable instructions, which makes it a program, which makes the act of writing it programming, which makes us programmers.
Being programmers that write and maintain VBA code does set us apart, mostly because the language isn’t going anywhere and the IDE is becoming more and more severely outdated and under-featured as years pass. Yet if the volume of VBA questions on Stack Overflow means anything, VBA is still very much alive, still very much being learned, and this is where Rubberduck and static code analysis comes in.
When I started learning about .NET and C# over a decade ago, there was this exciting new language feature they called LINQ for Language-INtegrated-Query where you could start querying object collections pretty much literally like you would a database, and it was awesome (still is!). In order to make this possible, the C# compiler and the .NET framework and runtime itself had to undergo some very interesting changes Jon Skeet covers in details, but the point is… the new syntax was a bit off-putting at first, and came with new and important implications (closures, deferred execution), and the company I worked for gave us all a ReSharper license, and that is how and when I discovered that thorough & accurate static code analysis tooling could be a formidable educational tool.
Maybe we don’t agree about Hungarian Notation, and that’s fine: Rubberduck wants you to be able to find it and rename it if that’s what you want to do, but you can mute that particular inspection anytime. But I believe the tool should tell you what Systems Hungarian notation is when it calls it out, and perhaps it should even explain what Apps Hungarian is and give examples, because Apps Hungarian notation absolutely is useful and meaningful (think o-for-OneBased, or src-for-Source and dst-for-Destination prefixes). But str-for-String, lng-for-Long, o-for-Object is different, in a bad kind of way.
Rubberduck flags obsolete code constructs and keywords, too. Global declarations, On Local Error statements, explicit Call statements, While...Wend loops, all have no reason to exist in brand new, freshly-written VBA code, and quick-fixes can easily turn them into Public declarations, On Error statements, implicit Call statements (without the Call keyword!), and Do While...Loop structures.
Rubberduck wants to push your programming towards objectively, quantitatively better code.
About Code Metrics
Rubberduck could count the number of lines in a procedure, and issue an inspection result when it’s above a certain configurable threshold. In fact, things are slowly falling into place for it to eventually happen. But we wouldn’t want you to just arbitrarily cut a procedure scope at 20 lines because an inspection said so! Rubberduck can measure line count, nesting levels, and cyclomatic complexity. These metrics can be used to identify problematic areas in a code base and methodically split up large complex problems into measurably much smaller and simpler ones.
Line Countsimply counts the number of lines. Eventually this would expand into Statements and Comments counts, perhaps with percentages; 10% comments is probably considered a good sign, for example. But no tool is going to tell you that ' increments i is a useless comment, and even the best tools would probably not tell the difference between a huge ' the following chunk of code does XYZ banner comment and an actually valuable comment. Common wisdom is to keep this line count metric down as much as possible, but one should not do this at the expense of readability.
Nesting Levels counts the number of… well, nesting levels. While nesting two For...Next loops to iterate a 2D array (or a Range of cells) down and across is probably reasonable, further nesting is probably better off made implicit through a procedure call. Rule of thumb, it’s always good idea to pull the body of a loop into its own parameterized procedure scope. Arrow-shaped code gets flattened, line count gets lower, and procedures become more specialized and have fewer reasons to fail that way.
Cyclomatic Complexity essentially calculates the number of independent execution paths in a given procedure (wikipedia). A procedure with a cyclomatic complexity above 5 is harder to follow than one with a complexity of 1 or 2, but it’s not uncommon for a “God procedure” with nested loops and conditionals to measure in the high 40s or above.
The code metrics feature will eventually get all the attention it deserves, but as with inspections the general idea is to highlight procedures that could be harder to maintain than necessary, and nudge our users towards:
Writing more, smaller, more specialized procedure scopes.
Passing parameters between procedures instead of using global variables.
Having more, smaller, more cohesive modules.
Navigating the VBE
You may or may not have noticed, but the Visual Basic Editor is nudging you in the exact opposite direction, because…
Having fewer, larger, more general-purpose procedures puts you in a scripting mindset.
Using globals instead of passing parameters around is perhaps a simpler thing to do.
Having fewer, larger, more general-purpose modules makes it simpler to share the code between projects, and arguably easier to find things in the Project Explorer.
If you’re actually writing a small script, you can and probably should absolutely do that.
But if you’re like me then you’ve been pushing VBA to do things it wasn’t really meant to do, and you’re maintaining actual applications that could just as well be written in any other language out there, but you’re doing it in VBA because [your reasons are valid, whatever they are].
And that’s kind of a problem, because the VBE seems to actively not want you to write proper object-oriented code: its navigation tooling indeed makes it very hard to work in a project with many small modules, let alone an OOP project involving explicit interfaces and high abstraction levels.
Rubberduck lifts pretty much all the IDE limitations that hinder treating a VBA project as more than just an automation script. Now you can have a project with 135 class modules, all neatly organized by functionality into folders that can contain any module type, so a UserForm can appear right next to the classes that use it, without needing to resort to any kind of ugly prefixing schemes. You can right-click on an abstract interface (or one of its members) and quickly find all classes that implement it. You get a Find symbol command that lets you quickly navigate to literally anything that has a name, anywhere in the project. Curious about the definition of a procedure, but don’t want to break your flow by navigating to it? Peek definition (currently only in pre-release builds) takes you there without leaving where you’re at.
The Peek Definition command pops a floating panel conveniently showing the source code for the user-defined module or member you’ve selected.Find all References shows all the places a given identifier is being used, and shows it in context so you can easily locate the specific usage you’re looking for – and then a double-click takes you there.The Find all Implementations command is incredibly useful in object-oriented projects that leverage polymorphism through abstract interfaces: quickly locate and navigate to any implementation of any interface (class or member).
The VBE’s Project Explorer aims to give you a bird’s eye view of your project, regrouping modules by module type which is great for a small script that can get away with a small number of components, but that makes it very hard to manage larger projects. With Rubberduck’s Code Explorer you get to drill down to member level, and regroup modules by functionality using an entirely customizable folder hierarchy:
The Code Explorer leaves the VBE’s Project Explorer in the dust, fair & square.
These navigational enhancements greatly simplify moving around a project of any size, although some of them might feel a bit overkill in a smaller project, and some of them are only useful in more advanced OOP scenarios. Still, having more than just a text-based search to look for things is very useful.
Guidelines
If there’s one single over-arching principle guiding everything else, it would have to be write code that does what it says, and says what it does. Everything else seems to stem from this. These are warmly recommended guidelines, not dogma.
Naming
Use PascalCase if you like. Use camelCase if you like. Consistency is what you want to shoot for, and in a case-insensitive language that only stores a single version of any identifier name it’s much easier and simpler to just use PascalCase everywhere and move on to more interesting things, like tabs vs spaces.
Avoid _ underscores in identifier names, especially in procedure/member names.
Causes compile errors with Implements.
Use meaningful names that can be pronounced.
Avoid disemvoweling (arbitrarily stripping vowels) and Systems Hungarian prefixing schemes.
A series of variables with a numeric suffix is a missed opportunity to use an array.
A good identifier name is descriptive enough that it doesn’t need an explainer comment.
Use a descriptive name that begins with a verb for Sub and Function procedures.
Use a descriptive name (a noun) for Property procedures and modules.
For object properties, consider naming them after the object type they’re returning, like Excel.Worksheet.Range returns a Range object, or like ADODB.Recordset.Fields returns a Fields object.
Appropriately name everything the code must interact with: if a rounded rectangle shape is attached to a DoSomething macro, the default “Rounded Rectangle 1” name should be changed to “DoSomethingButton” or something that tells us about its purpose. This includes all controls on a UserForm designer, too. CommandButton12 is useless; SearchButton is much better. Consider also naming the controls that don’t necessarily interact with code, too: future code might, and the author of that future code will appreciate that the bottom panel is named BottomPanel and not Label34.
Renaming
Naming is hard enough, renaming things should be easy. With Rubberduck’s Rename refactoring (Ctrl+Shift+R) you can safely rename any identifier once, and all references to that identifier automatically get updated. Without a refactoring tool, renaming a form control can only be done from the Properties toolwindow (F4), and doing this instantly breaks any event handlers for it; renaming a variable by hand can be tedious, and renaming a single-letter variable like a or i with a local-scope find/replace (Ctrl+H) can get funny if the scope has any comments. Rubberduck knows the exact location of every reference to every identifier in your project, so if you have a module with two procedures that each declare a localThing, when you rename the local variable localThing in the first procedure, you’re not going to be affecting the localThing in the other procedure. But if you rename CommandButton1 to OkButton, then CommandButton1_Click() becomes OkButton_Click().
Parameters & Arguments
Prefer passing values as parameters instead of bumping the scope of a variable to module-level, or instead of declaring global variables.
Pass parameters ByVal whenever possible.
Arrays and User-Defined Type structures cannot and should not be passed by value.
Objects are never passed anywhere no matter the modifier: it’s only ever (ByVal: a copy of) a pointer that gets passed around – and most of the time the intent of the author is to pass that pointer by value. A pointer is simply a 32-bit or 64-bit integer value, depending on the bitness of the process; passing that pointer ByRef (explicitly or not) leaves more opportunities for programming errors.
Use an explicit ByRef modifier whenever passing parameters by reference.
Consider specifying an out prefix to name ByRef return parameters.
Consider using named arguments for out-prefixed ByRef return parameters.
Comments
Use the single quote ' character to denote a comment.
Avoid line-continuing comments; use single quotes at position 1 of each line instead.
Consider having a @ModuleDescription annotation at the top of each module.
Consider having a @Description annotation for each Public member of a module.
Remove comments that describe what an instruction does, replace with comments that explain why an instruction needs to do what it does.
Remove comments that summarize what ablock of code does; replace with a call to a new procedure with a nice descriptive name.
Avoid cluttering a module with banner comments that state the obvious. We know they’re variables, or properties, or public methods: no need for a huge green comment banner to tell us.
Avoid cluttering a procedure scope with banner comments that split up the different responsibilities of a procedure: the procedure is doing too many things, split it up and appropriately name the new procedure instead.
Variables
Declare all variables, always. Option Explicit should be enabled at all times.
Declare an explicit data type, always. If you mean As Variant, make it say As Variant.
Consider using a Variant to pass arrays between scopes, instead of typed arrays (e.g. String()).
Pluralize these identifier names: it signals a plurality of elements/items much more elegantly than Pirate Notation (arr*) does.
Avoid Public fields in class modules; encapsulate them with a Property instead.
Consider using a backing user-defined Private Typestructure for the backing fields of class properties; doing so eliminates the need for a prefixing scheme, lets a property be named exactly as per its corresponding backing field, and cleans up the locals toolwindow by grouping the fields under a single module variable.
Limit the scope of variables as much as possible. Prefer passing parameters and keeping the value in local scope over promoting the variable to a larger scope.
Declare variables where you’re using them, as you need them. You should never need to scroll anywhere to see the declaration of a variable you’re looking at.
Late Binding
Late binding has precious little to do with CreateObject and whether or not a library is referenced. In fact, late binding happens implicitly rather easily, and way too often. Strive to remain in early-bound realm all the time: when the compiler / IntelliSense doesn’t know what you’re doing, you’re on your own, and even Option Explicit can’t save you from a typo (and error 438).
Avoid making a member call against Object or Variant. If a compile-time type exists that’s usable with that object, a local variable of that data type should be assigned (Set) the Object reference and the member call made early-bound against this local variable.
Taking an object presenting one interface and assigning it to another data type is called “casting”.
Of course explicit late binding is OK (As Object, no library reference, create objects with CreateObject instead of the New operator). Late binding is very useful and has many legitimate uses, but generally not when the object type is accessible at compile-time through a library reference.
Avoid the dictionary-access (aka “bang”) operator !, it is late-bound by definition, and makes what’s actually a string literal read like a member name, and any member call chained to it is inevitably late-bound too. Rubberduck can parse and resolve these, but they’re much harder to process than standard method calls.
Explicitness
Use explicit modifiers everywhere (Public/Private, ByRef/ByVal).
Declare an explicit data type, even (especially!) if it’s Variant.
Note: some object models define a hidden default member (e.g. Range.[_Default]) that redirects to another member depending on its parameterization. In such cases it’s best to invoke that member directly; for example use Range.Value as appropriate, but the hidden [_Default] member is better off not being invoked at all, for both readability and performance reasons.
Invoke parameterized default members implicitly when they are indexers that get a particular item in an object collection, for example the Item property of a Collection. Invoking them explicitly doesn’t hurt, but could be considered rather verbose.
Call is not a keyword that needs to be in your program’s vocabulary when you use expressive, descriptive procedure names that imply an action taking place.
Consider explicitly qualifying standard module member calls with the project (and module) name, including for standard and referenced libraries, especially in VBA projects that reference multiple object models.
Structured Programming (Procedural)
One macro/script per module. Do have it in a module rather than a worksheet’s code-behind.
Public procedure first, followed by parameterized Private procedures, in decreasing abstraction level order such that the top reads like a summary and the bottom like boring, small but specific operations.
You know it’s done right when you introduce a second macro/module and get to pull the small, low-abstraction, specific operations into Public members of a utility module, and reuse them.
Don’t Repeat Yourself (DRY).
Consider passing the relevant state to another procedure when entering a block of code. Code is simpler and easier to follow when the body of a loop or a conditional block is pulled into its own scope.
Avoid using error handling to control the flow of execution: the best error handling is no error handling at all, because assumptions are checked and things are validated. For example instead of opening a file from a parameter value, first verify that the file exists instead of handling a file not found error… but still handle errors, for any exceptional situations that might occur while accessing the file.
When it’s not possible to avoid error handling, consider extracting a Boolean function that swallows the expected error and returns False on failure, to simplify the logic.
Handle errors around all file and network I/O.
Never trust user inputs to be valid or formatted as expected.
Object Oriented Programming
In VBA/VB6 we get to go further than mere scripting and apply Object-Oriented Programming principles, probably more relevantly so in VB6 and larger VBA projects. For many years it has been drilled into our heads that VBA/VB6 cannot do “real” OOP because it doesn’t support inheritance. The truth is that there is much, much more to OOP than inheritance, and if you want to learn and apply OOP principles in your VBA/VB6 code, you absolutely can, and you absolutely should, and Rubberduck will absolutely help you do that.
Adhere to standard OOP best practices, they are general, language-agnostic concepts that couldn’t care less about the capabilities of VBA/VB6:
Single Responsibility Principle – each abstraction should be responsible for one thing.
Open/Closed Principle – write code that doesn’t need to change unless the purpose of the abstraction itself needs to change.
Liskov Substitution Principle – code should run the exact same execution paths regardless of the concrete implementation of a given abstraction.
Interface Segregation Principle – keep interfaces small and specialized, avoid a design that constantly needs new members to be added to an interface.
Dependency Inversion Principle – depend on abstractions, not concrete implementations.
Leverage composition where inheritance would be needed.
You cannot have parameterized constructors, but you still can leverage property injection in factory methods to inject instance-level dependencies.
Leverage method injection to inject method-level dependencies.
Avoid New-ing dependencies in-place, it couples a class with another, which hinders testability; inject the dependencies instead.
Use the New keyword in your composition root, as close as possible to an entry point.
The Workbook_Open event handler (Excel) is a possible entry point.
Macros (Sub procedures invoked from outside the code) are also valid entry points.
Let go of the idea that a module must control every last one of its dependencies: let something else deal with creating or dereferencing these objects.
Inject an abstract factory when a dependency cannot or should not be created at the composition root, for example if you needed to connect to a database and wish to keep the connection object as short-lived and tightly-scoped as possible.
Keep the default instance of a class stateless as much as possible. Actively protect/guard against accidental misuse by throwing a run-time error as necessary.
Use standard modules instead of a utility class with a @PredeclaredId, that never gets explicitly instantiated or used as an actual object.
User Interfaces
UI code is inherently object-oriented, and thus a UserForm should be treated as the object it wants to be. The responsibilities of a user interface are simple: display and collect data to/from the user, and/or offer a way to execute commands (which typically consume or otherwise manipulate the data).
Avoid working directly with the form’s default instance. New it up instead.
Form module / code-behind should be strictly concerned with presentation concerns.
Do implement UI logic in form’s code-behind, e.g. enable this control when this command says it can be executed, or show this label when the model isn’t valid, etc.
Consider creating a model class to encapsulate the form’s state/data.
Expose a read/write property for each editable field on the form.
Expose a read-only property for data needed by the controls (e.g. the items of a ListBox).
Controls’ Change handlers manipulate the model properties.
Expose additional methods and properties as needed for data/input validation.
Consider having an IsValid property that returns True when all required values are supplied and valid, False otherwise; use this property to enable or disable the form’s Accept button.
Avoid implementing any kind of side-effecting logic in a CommandButton‘s Click handler. A CommandButton should invoke a command, right?
In procedural code the command might be a Public Sub procedure in a standard module named after the form, e.g. a SomeDialogCommands module for a SomeDialog form.
In OOP the command might be a property-injected instance of a DoSomethingCommand class; the Click handler invokes the command’s Execute method and could pass the model as a parameter.
Consider implementing a presenter object that is responsible for owning and displaying the form instance; the Model-View-Presenter UI pattern is well documented, and like everything OOP, its concepts aren’t specific to any language or platform.
Caveat: Microsoft Access Data-Bound UI
VBA projects hosted in Microsoft Access can absolutely use UserForm modules too, but without Rubberduck you need to hunt down the IDE command for it because it’s hidden. Instead, in Access you mostly create Access Forms, which (being document modules owned by the host application) have much more in common with a Worksheet module in Excel than with a UserForm.
The paradigm is different in an Access form, because of data bindings: a data-bound form is inherently coupled with the underlying database storage, and any effort to decouple the UI from the database is working directly against everything Access is trying to make easier for you.
Treating an Access form the way one would treat a worksheet UI in Excel puts you in a bit of a different mindset. Imagine the Battleship worksheet UI implemented as an Access form: the game would be updating game state records in the underlying database, and instead of having code to pull the game state into the UI there would only need to be code to re-query the game state, and the data bindings would take care of updating the actual UI – and then the game could easily become multi-player, with two clients connecting to the database and sharing the same game state.
This is very fundamentally different than how one would go about getting the data into the controls without such data bindings. Binding the UI directly to a data source is perfectly fine when that data source happens to be running in the very same process your VBA code is hosted in: Access’ Rapid Application Development (RAD) approach is perfectly valid in this context, and its global objects and global state make a nice beginner-friendly API to accomplish quite a lot, even with only a minimal understanding of the programming language (and probably a bit of Access-SQL).
If we’re talking about unbound MS-Access forms, then it’s probably worth exploring Model-View-Presenter and Model-View-Controller architectures regardless: in such exploratory OOP scenarios the above recommendations can all hold.
UI Design
I’m not going to pretend to be a guru of UI design, but over the years I’ve come to find myself consistently incorporating the same elements in my modal forms, and it has worked very well for me so here we go turning that into general guidelines.
TopPanel is a Label control with a white background that is docked at the top and tall enough to comfortably fit short instructions.
BottomPanel is also a Label control, with a dark gray background, docked at the bottom and no more than 32 pixels in height.
DialogTitle is another Label control with a bold font, overlapping the TopPanel control.
DialogInstructions is another Label control overlapping the TopPanel control.
DialogIcon is an Image control for a 16×16 or 24×24 .bmp icon aligned left, at the same Top coordinate as the DialogTitle control.
OkButton, CancelButton, CloseButton, ApplyButton would be CommandButton controls overlapping the BottomPanel control, right-aligned.
The actual client area content layout isn’t exactly free-for-all, and I doubt it’s possible to come up with a set of “rules” that can apply universally, but we can try, yeah?
Identify each field with a label; align all fields to make it look like an implicit grid.
Seek visual balance; ensure a relatively constant margin on all sides of the client area, space things out but not too much. Use Frame controls to group ComboBox options.
Avoid making a complex form with too many responsibilities and, inevitably, too many controls. Beyond a certain complexity level, consider making separate forms instead of tabs.
Use Segoe UI for a more modern font than MS Sans Serif.
Do not bold all the labels.
Have a ToolTip string for the label of every field the user must interact with. If a field is required or demands a particular format/pattern, indicate it.
Consider toggling the visibility of a 16×16 icon next to (or even inside, right-aligned) input fields, to clearly indicate any data validation errors (have a tooltip string on the image control with the validation error message, e.g. “this field is required”, or “value cannot be greater than 100”).
Name. All. The. Things.
Use background colors in input controls only to strongly signal something to the user, like a validation error that must be corrected in order to move on. Dark red text over a light pink background makes a very strong statement.
Keep a consistent color scheme/palette and style across all of your application’s UI components.
This pretty much concludes the “guidelines” section (although I’ll quite probably be adding more to it), but since discussing unit testing and testability lines up with everything above…
Unit Testing
A unit test is a small, simple procedure that is responsible for 3 things:
Arrange dependencies and set expectations.
Act, by invoking the method or function under test.
Assert that the expected result matches the actual one.
When a unit test runs, Rubberduck tracks Assert.Xxxx method calls and their outcome; if a single Assert call fails, the test fails. Such automated tests are very useful to document the requirements of a particular model class, or the behavior of a given utility function with multiple optional parameters. With enough coverage, tests can actively prevent regression bugs from being inadvertently introduced as the code is maintained and modified: if a tweak breaks a test, you know exactly what functionality you broke, and if all tests are green you know the code is still going to behave as intended.
Have a test module per unit/class you’re testing, and consider naming the test methods following a MethodUnderTest_GivenAbcThenXyz, where MethodUnderTest is the name of the method you’re testing, Abc is a particular condition, and Xyz is the outcome. For tests that expect an error, consider following a MethodUnderTest_GivenAbc_Throws naming pattern. Rubberduck will not warn about underscores in test method names, and these underscores are safe because Rubberduck test modules are standard modules, and unit test naming recommendations usually heavily favor being descriptive over being concise.
What to test?
You want to test each object’s public interface, and treat an object’s private members as implementation details. You do NOT want to test implementation details. For example if a class’ default interface only exposes a handful of Property Get members and a Create factory method that performs property-injection and a handful of properties, then there should be tests that validate that each of the parameters of the Create method correspond to an injected property. If one of the parameters isn’t allowed to be Nothing, then there should be a guard clause in the Create method for it, and a unit test that ensures a specific error is being raised when the Create method is invoked with Nothing for that parameter.
Below is one such simple example, where we have 2 properties and a method; note how tests for the private InjectDependencies function would be redundant if the public Create function is already covered – the InjectDependencies function is an implementation detail of the Create function:
'@PredeclaredId
Option Explicit
Implements IClass1
Private Type TState
SomeValue As String
SomeDependency As Object
End Type
Private This As TState
Public Function Create(ByVal SomeValue As String, ByVal SomeDependency As Object) As IClass1
If SomeValue = vbNullString Then Err.Raise 5
If SomeDependency Is Nothing Then Err.Raise 5
Dim Result As Class1
Set Result = New Class1
InjectProperties Result, SomeValue, SomeDependency
Set Create = Result
End Function
Private Sub InjectProperties(ByVal Instance As Class1, ByVal SomeValue As String, ByVal SomeDependency As Object)
Instance.SomeValue = SomeValue
Set Instance.SomeDependency = SomeDependency
End Sub
Public Property Get SomeValue() As String
SomeValue = This.SomeValue
End Property
Public Property Let SomeValue(ByVal RHS As String)
This.SomeValue = RHS
End Property
Public Property Get SomeDependency() As Object
SomeDependency = This.SomeDependency
End Property
Public Property Set SomeDependency(ByVal RHS As Object)
Set This.SomeDependency = RHS
End Property
Private Property Get IClass1_SomeValue() As String
IClass1_SomeValue = This.SomeValue
End Property
Private Property Get IClass1_SomeDependency() As Object
IClass1_SomeDependency = This.SomeDependency
End Property
Note: the property injection mechanism doesn’t need a Property Get member on the Class1 interface, however not exposing a Property Get member for a property that has a Property Let (and/or Property Set) procedure, would leave the property as write-only on the Class1 interface. Write-only properties would be flagged as a design smell, so there’s a conundrum here: either we expose a Property Get that nothing is calling (except unit tests, perhaps), or we expose a write-only property (with a comment that explains its property injection purpose). There is no right or wrong, only a consistent design matters.
If we were to write unit tests for this class, we would need at least:
One test that invokes Class1.Create with an "" empty string for the first argument and fails if error 5 isn’t raised by the procedure call.
One test that invokes Class1.Create with Nothing for the second argument and fails if error 5 isn’t raised by the procedure call.
One test that invokes Class1.Create with valid arguments and fails if the returned object is Nothing.
One test that invokes Class1.Create with valid arguments and fails if the Class1.SomeValue property doesn’t return the value of the first argument.
One test that invokes Class1.Create with valid arguments and fails if the Class1.SomeDependency property doesn’t return the very same object reference as was passed for the second argument.
One test that invokes Class1.Create with valid arguments and fails if the IClass1.SomeValue property doesn’t return the same value as Class1.SomeValue does.
One test that invokes Class1.Create with valid arguments and fails if the IClass1.SomeDependency property doesn’t return the same object reference as Class1.SomeDependency does.
Obviously that’s just a simplified example, but it does perfectly illustrate the notion that the answer to “what to test?” is simply “every single execution path”… of every public member (private members are implementation details that are invoked from the public members; if they specifically need tests, then they deserve to be their own concern-addressing class module).
What is testable?
Without the Property Get members of Class1 and/or IClass1, we wouldn’t be able to test that the Create method is property-injecting SomeValue and SomeDependency, because the object’s internal state is encapsulated (as it should be). Therefore, there’s an implicit assumption that a Property Get member for a property-injected dependency is returning the encapsulated value or reference, and nothing more: by writing tests that rely on that assumption, we are documenting it.
Now SomeDependency might be an instance of another class, and that class might have its own encapsulated state, dependencies, and testable logic. A more meaty Class1 module might have a method that invokes SomeDependency.DoSomething, and the tests for that method would have to be able to assert that SomeDependency.DoSomething has been invoked once.
If Class1 wasn’t property-injecting SomeDependency (for example if SomeDependency was being New‘d it up instead), we would not be able to write such a test, because the outcome of the test might be dependent on a method being called against that dependency.
A simple example would be Class1 newing up a FileSystemObject to iterate the files of a given folder. In such a case, FileSystemObject is a dependency, and if Class1.DoSomething is newing it up directly then every time Class1.DoSomething is called, it’s going to try and iterate the files of a given folder, because that’s what a FileSystemObject does, it hits the file system. And that’s slow. I/O (file, network, …and user) is dependent on so many things that can go wrong for so many reasons, having it interfere with tests is something you want to avoid.
The way to avoid having user, network, and file inputs and outputs interfere with the tests of any method, is to completely let go of the “need” for a method to control any of its dependencies. The method doesn’t need to create a new instance of a FileSystemObject; what it really needs is actually a much simpler any object that’s capable of returning a list of files or file names in a given folder.
So instead of this:
Public Sub DoSomething(ByVal Path As String)
With CreateObjet("Scripting.FileSystemObject")
' gets the Path folder...
' iterates all files...
' ...
End With
End Sub
We would do this:
Public Sub DoSomething(ByVal Path As String, ByVal FileProvider As IFileProvider)
Dim Files As Variant
Files = FileProvider.GetFiles(Path)
' iterates all files...
' ...
End Sub
Where IFileProvider would be an interface/class module that might look like this:
Option Explicit
'@Interface
'@Description "Returns an array containing the file names under the specified folder."
Public Function GetFiles(ByVal Path As String) As Variant
End Function
That interface might very well be implemented in a class module named FileProvider that uses a FileSystemObject to return the promised array.
It could also be implemented in another class module, named TestFileProvider, that uses a ParamArray parameter so that unit tests can take control of the IFileProvider dependency and inject (here by method injection) a TestFileProvider instance. The DoSomething method doesn’t need to know where the file names came from, only that it can expect an array of existing, valid file names from IFileProvider.GetFiles(String). If the DoSomething method indeed doesn’t care where the files came from, then it’s adhering to pretty much all OOP design principles, and now a test can be written that fails if DoSomething is doing something wrong – as opposed to a test that might fail if some network drive happens to be dismounted, or works locally when working from home but only with a VPN.
The hard part is obviously identifying the dependencies in the first place. If you’re refactoring a procedural VBA macro, you must determine what the inputs and outputs are, what objects hold the state that’s being altered, and devise a way to abstract them away and inject these dependencies from the calling code – whether that caller is the original entry point macro procedure, or a new unit test.
Mocking
In the above example, the TestFileProvider implementation of the IFileProvider dependency is essentially a test stub: you actually write a separate implementation for the sole purpose of being able to run the code with fake dependencies that don’t incur any file, network, or user I/O. Reusing these stubs in “test” macros that wire up the UI by injecting the test stubs instead of the actual implementations, should result in the application running normally… without hitting any file system or network.
With mocks, you don’t need to write a “test” implementation. Instead, you configure an object provided by a mocking framework to behave as the method/test needs, and the framework implements the mocked interface with an object that can be injected, that verifiably behaves as configured.
Sounds like magic? A lot of it actually is, from a VBA/VB6 standpoint. Many tests in Rubberduck leverage a very popular mocking framework called Moq. What we’re going to be releasing as an experimental feature is not only a COM-visible wrapper around Moq. The fun part is that the Moq methods we need to use are generic methods that take lambda expressions as parameters, so our wrapper needs to expose an API VBA code can use, and then “translate” it into member calls into the Moq API, but because they’re generic methods and the mocked interface is a COM object, we essentially build a .NET type on the fly to match the mocked VBA/COM interface, so that’s what Moq actually mocks: a .NET interface type Rubberduck makes up at run-time from any COM object. Moq uses Castle Windsor under the hood to spawn instances of proxy types – made-up actual objects that actually implement one or more interfaces. Castle Windsor is excellent at what it does; we use CW to automate dependency injection in Rubberduck (a technique dubbed Inversion of Control, where a single container object is responsible for creating all instances of all objects in the application in the composition root; that’s what’s going on while Rubberduck’s splash screen is being displayed).
There is a problem though: CW seems to be caching types with the reasonable but still implicit assumption that the type isn’t going to change at run-time. In our case however, this means mocking a VBA interface once and then modifying that interface (e.g. adding, removing, or reordering members, or changing a member signature in any way) and re-running the test would still be mocking the old interface, as long as the host process lives. This isn’t a problem for mocking a Range or a Worksheet dependency, but VBA user code is being punished here.
Verifiable Invocations
Going back to the IFileProvider example, the GetFiles method could be configured to return a hard-coded array of bogus test strings, and a test could be made to turn green when IFileProvider.GetFiles is invoked with the same specific Path parameter value that was given to Class1.DoSomething. If you were stubbing IFileProvider, you would perhaps increment a counter every time IFileProvider_GetFiles is invoked, and expose that counter with a property that the test could Assert is equal to an expected value. With Moq, you can make a test fail by invoking a Verify method on the mock itself, that verifies whether the specified method was invoked as configured.
A best practice with mocking would be to only setup the minimal amount of members to make the test work, because of the performance overhead: if a mocked interface has 5 methods and 3 properties but the method under test only needs 2 of these methods and 1 of these properties, then it should only setup these. Verification makes mocking a very valuable tool to test behavior that relies on side-effects and state changes.
The best part is that because VBA is COM, then everything is an interface, so if you don’t have an IFileProvider interface but you’re still passing a FileProvider object as a dependency, then you can mock the FileProvider directly and don’t need to introduce any extra “just-for-testing” IFileProvider interface if you don’t already have one.
I’m going to stop here and just publish, otherwise I’ll be editing this post forever. So much is missing…
Version 2.5.1 was released August 22, 2020. Since then, the installer was downloaded over 11,600 times; we are now 420 commits and 650 modified files later, and the time has come to deliver all that work into a convenient little installer package and move on to the next dev/release cycle.
What’s New?
If you’ve kept up with latest pre-release builds (especially in the last few weeks), nothing much. If you’ve been patiently waiting for the next release, you’re in for a treat!
The first thing you’ll probably notice is the shiny new splash screen design:
It’s the same old yellow splash made with Paint.NET, with a tiled reflection distortion effect against the background, a semi-transparent white bottom panel, and a finer font. Do you like it?
50-some issues labelled “bug” were closed between 2020-08-22 and mid-April 2021, many of them thanks to flicking the switch on leveraging our internal ITypeLib API for user code – thanks to earlier invaluable contributions from the amazing Wayne Phillips (vbWatchdog, twinBASIC), Rubberduck is now able to tap into the actual in-memory COM type library compiled from the VBA code and, eventually, fill the remaining the gaps in Rubberduck’s understanding of the code: Rubberduck now understands enough to be able to tell that ThisWorkbook has a _Workbook subtype, and that Sheet1 has a _Worksheet subtype, …and that’s enough to identify the ThisWorkbook module at long last, and as a result Rubberduck’s ImplicitActiveSheetReference and ImplicitActiveWorkbookReference inspections get to work exactly as intended, and the door is now opened for so many interesting things…
New Inspections
A Rubberduck release wouldn’t be a Rubberduck release without at least a handful of new inspections. The IllegalAnnotation inspection is being replaced by InvalidAnnotation, UnrecognizedAnnotation, and together with the new AnnotationInIncompatibleComponentType inspection they allow Rubberduck to better convey exactly what’s wrong with a given “illegal” annotation comment.
Some annotations cannot be used in certain types of modules. For example, attribute-related annotations cannot be used in document modules (because Rubberduck cannot import back the modified modules), and a @TestModule annotation is only meaningful in a standard module.
Note that the @Description, @ModuleDescription and @VariableDescription annotations do work in document modules now, because Rubberduck is now reading docstrings off annotations rather than hidden attributes.
Code in the ThisWorkbook module (Excel) referring to members of the Workbook class, have an implicit Me qualifier. This makes an unqualified Worksheets(1) retrieval in ThisWorkbook refer to ThisWorkbook.Worksheets(1), but an identical statement in any other module would be (implicitly) referring to ActiveWorkbook. By qualifying such member calls with Me, the intent is clarified.
Code in a worksheet module (Excel) referring to members of the Worksheet class, have an implicit Me qualifier. This makes an unqualified Range member call in the Sheet2 module refer to Sheet2, but an identical statement in any other module would be (implicitly) referring to ActiveSheet. By qualifying such member calls with Me, the intent is clarified.
Flags unbound annotations; that is, annotation comments that were correctly parsed as Rubberduck annotations but that could not be associated with a target element. This would happen when a module annotation is used in local scope, or a member annotation at module level. This inspection only flags annotation comments that parsed as Rubberduck annotations.
The RHS/Value parameter of a Property Let procedure is always passed by value. As such, an explicit ByRef modifier on such a parameter definition is misleading. From MS-VBAL (VBA language specifications) section 5.3.1.7 Property Declarations:
§ If the <value-param> of a <property-LHS-declaration> does not have a <parameter-mechanism> element or has a <parameter-mechanism> consisting of the keywordByRef, it has the same meaning as if it instead had a <parameter-mechanism> element consisting of the keywordByVal. § The <value-param> of a <property-LHS-declaration> always has the runtime semantics of a ByVal parameter.
This inspection flags comments that parsed like a Rubberduck annotation, but aren’t recognized or supported. It picks up typos in Rubberduck annotations, and annotation-like comments that aren’t Rubberduck annotations but parse as such. Splicing this specific scenario from other invalid annotations is particularly useful when you want to mute inspection results for non-Rubberduck annotations while still validating the supported ones.
New Quick Fixes
This release also introduces a handful of new quick-fixes:
This fix is now available for ProcedureNotUsed inspection results in standard and document modules; it simply annotates a member with the new @EntryPoint annotation which specifically instructs ProcedureNotUsed to ignore that member. Use this quick-fix for UDFs and macro procedures that are attached to document objects and don’t need an Excel hotkey. If your project is hosted in an Excel workbook, macros annotated with @ExcelHotkey are also considered as entry points now.
VariableTypeNotDeclared inspection results could always be “fixed” by making the declared type an explicitVariant; this new quick-fix makes Rubberduck infer the declared type from usage where possible, which is objectively awesome.
This new quick-fix is available for the new implicit containing workbook/worksheet reference inspections, making the reference to the containing module explicit.
The Write-Only Property inspection gets a new quick-fix with this release; this iteration does not try to infer the backing field, so further manual edits are needed, but it’s a start.
New UI Language: Italian
Thanks to a timely contribution by @PhilCattivocaratere, we are thrilled to announce that this release introduces Italian as a UI language:
Every single UI string in Rubberduck comes from a localized resource file. Translating all the resources for a new language can take 3-5 hours, and then it’s only a matter of keeping the translations up-to-date by creating a small pull request when new resource strings are added for new features.
In a nutshell
Here’s a quick summary of the most significant pull requests and commits merged this cycle:
Encapsulate Field enhancements
We are now leveraging our internal ITypeLib API
We are now building Rubberduck with the latest version of Visual Studio 2019
Precompiler directives now parse correctly with line continuations
Internal CodeBuilder API honors indenter settings when generating code
Fixed a number of issues with name conflict validation
Test methods now support a @TestIgnore annotation to ignore a test
Specific projects can now be ignored by the parser
Users no longer need to accept the GPLv3 as if it were an End User License Agreement (EULA)
Custom templates extensions is changing from .rdt to .template
Implicit Variant inspection quick-fix will now infer the best type from usage instead of just making the variable an explicitVariant
For...Next loop variables no longer trigger a variable not used inspection
Implicit Public Member inspection will now flag Enum types and Type structures
Branch “master” was renamed to “main”
New Property Group indenter settings
Arrays declared with ReDim now correctly resolve the declared type
@Description, @VariableDescription, and @ModuleDescription can now be used in document modules (cannot be synchronized)
Documentation strings are now read from annotations when missing from attributes
Start menu link to website now uses https
Fixed context menu positioning
New @EntryPoint annotation marks a standard or document module member as invoked from outside the code; as such the Procedure Not Used inspection will no longer flags members annotated with @EntryPoint or @ExcelHotkey (Excel only).
Several other opportunistic fixes left & right, improved overall stability.
Shiny new splash screen; debug builds now indicate “debug” instead of a meaningless local build number (build version# is controlled by the AppVeyor CI build server; local builds are all .0).
Expand/collapse all in Code Explorer
Rubberduck CommandBar label will now show the corresponding parameter declaration for a selected argument, and Find all References will now include arguments at call sites for parameter declarations (previous versions would only count named arguments).
Find Symbol navigation tool works again.
Find all References search results will now highlight the target reference in its context.
Added Italian UI resources.
Possible (Silent) Crash on Exit
I haven’t personally experienced it in a long time in Excel, but Rubberduck may run into issues tearing down, sometimes causing an AccessViolationException when it unloads, which can either crash the host process or leave it hanging as a ghosted process that will interfere with reloading: verify that the host process (e.g. ACCESS.EXE) has shut down completely using Task Manager when you close everything, and make sure to kill any such ghosted processes before loading Rubberduck in a new process.
Sounds familiar? If you’ve been following the project all along, you probably remember similar behavior in earlier releases – at one point during this development cycle we thought the problem was finally under control, but the cure was worse than the disease and there was a chance that the host document / project gets completely corrupted and impossible to open in the VBE: because we think it’s much better to sometimes crash on teardown than to corrupt our users’ host documents forever, we have reverted that “fix” and will have to come up with something else.
What’s Next?
Lots of good stuff, including a new peek definition command to the code pane, Code Explorer, and the VBE’s own Project Explorer‘s context menus – the feature was developed too late to make the cut for this release, but will be available in 2.5.2.x pre-release builds very soon:
Peek Definition commands pop a panel that shows you the syntax-highlighted source code for a type or member. The pop-up panel can then be dragged around to keep it in sight while editing.
In the Unit Testing department, a mocking framework is about to debut as an experimental feature with a number of technical limitations.
I’m going to be turning my attention towards code path analysis this cycle; this internal API is needed to implement the more advanced inspection ideas, and an Extract Method refactoring needs it too.
Most of the time, we don’t need any global variables. State can usually be neatly encapsulated in an object, and a reference to this object can easily be passed as an argument to any procedure scope that needs it. But global scope is neither a necessary evil, nor necessarily evil. Like many things in programming, it’s a tool, and like many other tools, misusing it can cause pain.
The VBA code and host Excel workbook accompanying this article can be found on GitHub.
What is Global Scope?
When we declare a variable inside a procedure, we call it a “local variable” in reference to its scope being local to the procedure. “Module variables” are accessible within any procedure scope within the module they’re declared in. Public members of private modules (and Friend members of public modules) are only accessible within the project they live in, and Public members of public modules are global and can be accessed from other projects.
The different scopes of VBA: Global, project, module, and local.
Because in VBA class modules are private by default, and a public class is only PublicNotCreatable (as in, a referencing project cannot create a New instance of a class, factory methods must be provided), and also because “actually global” is in reality slightly more complicated than that (the VB_GlobalNamespace attribute is always going to be False for a VBA class), for the sake of simplicity when I talk about “global scope” and “globals” in this article, I’m treating global and project scopes as one and the same – but it’s important to know the difference, especially more so in scenarios where a VBA/Excel add-in/library is being referenced by other VBA projects, where a tidy public API is handy.
Keywords Rubberduck recommends using the Dim keyword only in local scope, and to use the Private keyword to declare module-level variables. It also recommendsusing Public over Global, because nothing is really “global” in VBA and that makes the deprecated keyword potentially confusing. The Global keyword really means Public in VBA, and should be avoided.
Picture the VBA runtime executing some macro procedure and some variable needs to be incremented by 1. Scope determines whether that variable identifier is referring to a local, module, or global declaration. Accessibility is how we use code to restrict scope, using keywords like Private, Public, or Friend: if the variable identifier exists in a public module but is declared with the Private keyword, then it’s inaccessible and not in scope for the procedure we’re in.
So in search for the variable’s declaration we look for a local scope declaration by that name. If there isn’t any, we look for a module scope declaration for that name. Not there? We look at everything we can see in project scope. If we still haven’t found it then, we look for the declaration in the referenced libraries and projects, in priority order (so, the VBA standard library, then the host application’s own object model library, then everything else).
That’s scoping. Scopes and accessibility are closely related, but they’re different things. Think of accessibility as a tool to shape your private and public interfaces and APIs, keeping in mind that in VBA all module members are implicitly Public unless their declaration states otherwise.
Globals and Testability
Global variables are very useful: having a piece of data that is accessible from anywhere in the code does have its advantages. Used wisely, globals can very elegantly address cross-cutting concerns. Instead of having every method responsible for its own logging, or instead of passing a Logger instance to every method, each scope can access a single global Logger object (or invoke the same Log utility procedure), and there really isn’t any problem with that, …until you realize that your unit tests are all needlessly writing logs to some file under C:\Dev\VBA because the global logger is doing its job whether or not the code invoking it is being executed from a test runner… and this is making tests run code that isn’t related to these tests’ purpose: if there’s a bug in the logger code, it’s a test about the logger code that should be failing, not every single other test that couldn’t care less for the logging functionality.
From a testability standpoint, code with global dependencies can be difficult, if not impossible to test. In the case of a global Logger dependency, the logger’s interface would need to expose some kind of “kill switch” that tests can invoke to disable logging… but then modifying an otherwise perfectly good interface for the sake of making the object aware of whether it’s being invoked from a test or not, isn’t ideal at all (we’ll see why in a bit).
This Logger is a good example of a legitimate global service, but it’s “user code” that could always be pragmatically modified to accommodate testing. What about code that depends on global-scope services that aren’t “user code”?
Treating the Excel Object Model as a Dependency
Imagine needing to write tests for user-defined functions (UDF) that store a number of values in a global Dictionary and then schedule a macro that then runs (asynchronously!) and sends these values over to some web API that returns data that then ends up on the worksheet, underneath the calling UDF; the functions have dependencies on Application.Caller and Application.OnTime: we don’t own the Application global object, and we can’t modify its code to accommodate testing – what then?
Writing tests for a UDF is normally trivial: the function takes inputs, computes a result, and then returns it. Tests can supply various inputs and run the function through all kinds of cases and assert that it handles them correctly, by simply comparing its return value with what’s expected, and exceptional edge cases can have tests asserting that the expected error is thrown.
Writing tests for a side-effecting UDF that temporarily stores data in global scope is a lot more challenging, for many reasons. Remember, unit tests:
Should reliably produce the same outcome regardless of any external factors;
Should be fast, and not involve any I/O or network activity;
Should be able to be executed individually or in any given order without affecting outcome;
Should be able to be executed concurrently (at least in theory – VBA won’t run concurrent code).
With state shared between the tests, we have to be careful to correctly setup and clean-up that state before & after each test, so that each test gets a fresh canvas in a controlled environment… and then we can live with VBA unit tests that would likely break if executed concurrently, because VBA can’t run them concurrently anyway.
Testing Untestable Things
Back to this not-so-crazy UDF scenario with the Application.OnTime hack: it wouldn’t be acceptable for a test to literally wait for Excel to decide it’s time to invoke a macro, not any more than a test should be sending any actual HTTP requests (although that would be very a good way to actually be testing an API’s rate limits and get acquainted with throttling, I guess), let alone parse and process an actual HTTP response.
Such a user-defined function involves too many moving parts soldered together to be testable: making the code testable involves making the parts moving parts again, and yes it involves a proverbial blowtorch and lots of proverbial sparks flying everywhere.
Refactoring code to make it testable is a lot of fun, but the first step is, ideally, to fully grasp what’s going on and why.
If you aren’t familiar with using Application.OnTime in user-defined functions (only indirectly, because Application.OnTime calls, like run-time errors and many other members in the Excel object model, get “swallowed” when Excel is evaluating a UDF), it’s a pretty cool process that goes like this:
The calling cell contains the UDF’s return value just before the macro gets asynchronously invoked and produces its own output.
So if a UDF stored its arguments as key/value pairs in a global-scope dictionary, if all goes well and according to plan, the macro that runs a moment later gets to consume this data.
By storing the Application.Caller cell object reference in global scope, the side-effecting macro gets to know where to put its results table. There’s always the possibility that a second UDF overwrites this global state during the split-second between the moment a first UDF writes it and the moment the scheduled asynchronous read of this global state actually happens: it’s important to keep in mind that Ambient Context does not inherently address this particular problem; the state is still global and mutable from anywhere in the code, and there is never any guarantee that any scope will run to completion before the VBA runtime decides it’s an asynchronous callback’s turn to run.
The Application.Caller member isn’t going to return a Range reference when it’s not a worksheet cell invoking the function, we can’t afford to wait for Application.OnTime, and we’d like to avoid actually invoking any Win32 API functions during a test. That UDF simply isn’t testable as-is.
The solution is to introduce an abstraction to wrap the Application members we need, and make the side-effecting UDFs depend on that abstraction instead of invoking Application members directly.
AbstractionThe untestable code might look something like this:
Public Function SideEffectingUDF(ByVal FirstParameter As String, ByVal SecondParameter As Long) As Variant
Set SomeGlobalRange = Application.Caller.Offset(RowOffset:=1)
With SomeGlobalDictionary
.Clear
.Add "FirstParameter", FirstParameter
.Add "SecondParameter", SecondParameter
End With
ScheduleMacro
End Function
Where ScheduleMacro involves a Win32 API call to schedule the execution of an Execute procedure that handles the Application.OnTime scheduling of the actual side-effecting procedure.
We want to be able to write a test that invokes this SideEffectingUDF function, and determines whether Application.Caller was invoked: Application.Caller is a dependency here, and for the test to be able to fulfill its purpose we must find a way to inject the dependencies so they can be controlled by the test, from outside the function.
Note how narrow such a test would be: it asserts that the UDF gets the Application.Caller reference, nothing more. Other tests would be similarly narrow, but for other things, and we don’t want a failing Application.Caller member call to interfere with these other tests by throwing error 91 before the test gets to do its thing. Whether or not we need to know if a UDF does or does not invoke Application.Caller, we still need a way to abstract the dependency away, to stub it.
You may be thinking “oh that’s easy” and be tempted go down this path:
Public Function SideEffectingUDF(ByVal FirstParameter As String, ByVal SecondParameter As Long) As Variant
If TypeOf Application.Caller Is Excel.Range Then
' caller is a worksheet cell
Set ThatGlobalCell = Application.Caller.Offset(RowOffset:=1)
With ThatGlobalDictionary
.Clear
.Add "FirstParameter", FirstParameter
.Add "SecondParameter", SecondParameter
End With
ScheduleMacro "SideEffectingMacro"
Else
' caller is a unit test
Set ThatGlobalCell = Sheet1.Cells(1, 1) ' tests can read as "Application.Caller was invoked"
With ThatGlobalDictionary
.Clear
.Add "FirstParameter", FirstParameter
.Add "SecondParameter", SecondParameter
End With
SideEffectingUDF = True ' tests can read this as "macro was scheduled"
End If
End Function
While it does solve the problem of avoiding to involve Application.Caller and actually scheduling the macro in tests, there are several reasons why this is a terrible idea:
Function now has a higher Cyclomatic Complexity metric by virtue of now needing more execution paths to accomplish the same thing: the code is objectively and measurably more complex now, on top of being repetitive (copying & pasting any code is usually a sign something is off!).
Tests are no longer executing the same code as normal execution does, which means tests are now testing code that only exists because there are tests: the normal execution path remains untested, and that makes the tests worthless busy-work.
Tests now need to be making assumptions about how the function is implemented, which effectively casts the code into concrete instead of making it simpler & safer to modify.
Dependencies should be abstractions, and code should be working with these abstractions without regards to their actual implementation: code that acts differently when the runtime type of an abstraction is X vs when it’s Y, violates the Liskov Substitution Principle, the “L” of “SOLID” that essentially states that all implementations of a given abstraction should be treated the same.
The killer is the second bullet: if the sole purpose of a test is to determine whether Application.Caller was invoked, and the UDF says “oh we’re in a test, here yeah everything is all right, see”, then a UDF that does nothing but returning True would pass that test, and that is why the test is useless, as is the code duplication.
When we write a test whose purpose is to determine whether the Application.Caller dependency was invoked, the test should FAIL when it isn’t, otherwise that test is just as good as a deleted one.
Now picture the UDF looking like this instead:
Public Function SideEffectingUDF(ByVal FirstParameter As String, ByVal SecondParameter As Long) As Variant
With AppContext.Current
Set .Target = .Caller.Offset(RowOffset:=1)
.Property("FirstParameter") = FirstParameter
.Property("SecondParameter") = SecondParameter
.ScheduleMacro
End With
End Function
The UDF now only has one dependency, AppContext.Current, which is global state by virtue of being accessible from the default instance of the AppContext class; we’re tightly coupled with the AppContext class, but only because we specifically want to access global state in a controlled manner, and the rest of the function is working against the IAppContext abstraction. The state that was formerly a Range and a Dictionary globally-scoped declaration is now properly encapsulated in an object, and the “current” AppContext is coming into existence from outside the UDF scope (but still from within our own code), which is exactly what we want: now unit tests get to inject a TestContext instead of manipulating global state.
So how do we get there?
Implementation
The basic idea is to pull our dependencies from global scope, encapsulate them in a class module, …and then making an instance of that class an “ambient context” that’s still globally accessible, but that introduces the necessary abstraction needed to make that UDF fully testable.
We want to leverage the default instance of the AppContext class, so we’re going to need an AppContext class with a @PredeclaredId annotation and a Current property getter that returns some IAppContext instance. If you’re familiar with factory methods this will feel a bit like something you’ve already seen:
'@PredeclaredId
Option Explicit
Implements IAppContext
Private Type TState
Factory As IAppContextFactory
Current As IAppContext
'...
End Type
Private This As TState
'@Description "Gets the current (or default) context."
Public Property Get Current() As IAppContext
Errors.GuardNonDefaultInstance Me, AppContext, TypeName(Me)
If This.Current Is Nothing Then
Set This.Current = This.Factory.Create
Errors.GuardNullReference This.Factory, TypeName(Me), "IAppContextFactory.Create returned Nothing."
End If
Set Current = This.Current
End Property
Private Property Get IsDefaultInstance() As Boolean
IsDefaultInstance = Me Is AppContext
End Property
Private Sub Class_Initialize()
If IsDefaultInstance Then
'must initialize context with sensible defaults:
Set This.Factory = New AppContextFactory
Set This.TimerProvider = New TimerProvider
Else
Set This.Properties = New Scripting.Dictionary
'we want all instances to have the same provider instance:
Set This.TimerProvider = AppContext.TimerProvider
End If
End Sub
We don’t normally want Property Get procedures to be side-effecting, but with an Ambient Context what we want is to yield a cached instance of the context class, so when no instance already exists, the getter caches the created object so it’s readily available next time, making it accessible from anywhere in the project (aka “global”).
Abstract Factory
The default instance of the AppContext class does not know what the actual runtime type of the Current context is, and this polymorphism is the cornerstone making it all work: the Current property getter is responsible for caching the new context instance, but not for actually creating it. That’s the job of an abstract factory (the IAppContextFactory dependency) that we conveniently initialize to a concrete factory type that creates instances of… the AppContext class.
Why involve an abstract factory to create an instance of the class we’re in, you might ask? Because that’s only the default implementation, and with ability to Set the Factory reference from outside the class, tests can inject a different factory implementation, say, this one named TestContextFactory:
'@Folder "Tests.Stubs"
'@ModuleDescription "A factory that creates TestContext instances."
Option Explicit
Implements IAppContextFactory
Private Function IAppContextFactory_Create() As IAppContext
Set IAppContextFactory_Create = New TestContext
End Function
Meanwhile the actual UDFs would be using this AppContextFactory implementation by default:
'@Folder "AmbientContext"
'@ModuleDescription "A factory that creates AppContext instances."
Option Explicit
Implements IAppContextFactory
Private Function IAppContextFactory_Create() As IAppContext
Set IAppContextFactory_Create = New AppContext
End Function
The AppContext.Current property will happily cache an instance of any class whatsoever, as long as it implements the IAppContext interface. The abstract factory pattern allows us to spawn an instance of a class at run-time, of which we don’t necessarily know the actual “concrete” type at compile-time.
In other words just by reading the UDF code, there is no way to tell whether AppContext.Current is going to be an AppContext or a TestContext instance, and that is exactly what we want.
What this abstraction achieves, is the decoupling that is necessary for a test to be able to inject a TestContextFactory and take control of everything UDFs can do with an IAppContext object.
Context State
We know the context needs to wrap Application.Caller and Application.OnTime functionality. We know we need a Target cell, we need some Properties in an encapsulated Scripting.Dictionary. If we crammed all that into a single interface, we would get a somewhat crowded IAppContext interface that doesn’t quite adhere to the Interface Segregation Principle and Open/Closed Principle guidelines.
By abstracting away the macro-scheduling functionality into its own IAppTimer interface, and making that interface an abstract dependency of the context class, we can stub that abstract dependency and write tests for the logic of the context class itself. Without this extra step, the context can be stubbed to test the code that uses it, but the macro-scheduling bits would remain untestable.
Treating IAppTimer as a dependency of the context makes the IAppContext interface look like this:
'@Folder "AmbientContext.Abstract"
'@ModuleDescription "Encapsulates the data and macro invocation mechanism for a side-effecting UDF."
'@Interface
Option Explicit
'@Description "Gets the cell that invoked the currently-running user-defined function (UDF), if applicable; Nothing otherwise."
Public Property Get Caller() As Range
End Property
'@Description "Gets or sets the target reference cell that the side-effecting macro shall use."
Public Property Get Target() As Range
End Property
Public Property Set Target(ByVal Value As Range)
End Property
'@Description "Gets or sets a named value representing data passed between the UDF and the side-effecting macro."
Public Property Get Property(ByVal Name As String) As Variant
End Property
Public Property Let Property(ByVal Name As String, ByVal Value As Variant)
End Property
'@Description "Gets an array of all property names."
Public Property Get Properties() As Variant
End Property
'@Description "Gets or sets the IAppTimer dependency."
Public Property Get Timer() As IAppTimer
End Property
Public Property Set Timer(ByVal Value As IAppTimer)
End Property
'@Description "Clears all held state."
Public Sub Clear()
End Sub
Note that we’re not exposing the dictionary itself: rather we expose an indexed property to get/set the key/values, then by exposing the dictionary keys, the calling code gets to do everything it needs to do, without ever directly interacting with a Scripting.Dictionary, a bit as if the AppContext class were a custom collection.
Now, there’s something special about the IAppTimer dependency: we absolutely cannot have each context instance spawn timers willy-nilly, because a leaking Win32 timer is a nice way to send Excel up in flames. Yet, we need each context instance to be able to access the same IAppTimer reference.
A good way to solve this is by introducing a Provider mechanism. The interface looks like this:
'@ModuleDescription "A service that ensures all clients get the same IAppTimer instance."
'@Interface
Option Explicit
'@Description "Gets an IAppTimer instance."
Public Property Get Timer() As IAppTimer
End Property
What I’m calling a “provider” here is exactly the same mechanism that provides the IAppContext instance (a Property Get procedure that gets a cached object or creates the object and caches it), except no abstract factory needs to get involved here. The class also makes a very convenient place to put the name of the Win32 callback macro procedure:
Option Explicit
Implements ITimerProvider
Private Const MacroName As String = "Execute"
Private Property Get ITimerProvider_Timer() As IAppTimer
Static Instance As AppTimer
If Instance Is Nothing Then
Set Instance = New AppTimer
Instance.MacroName = MacroName
End If
Set ITimerProvider_Timer = Instance
End Property
TimerProvider the only object that creates a New AppTimer: as a result, every AppContext instance created from this factory is going to use the same IAppTimer reference, and if we need to write tests for AppContext we can inject a TestTimerProvider that returns a TestTimer.
Note that the “provider” mechanism is an implementation detail of AppContext: the TestContext doesn’t need this, because it just initializes itself with a TestTimer, while AppContext initializes itself with a TimerProvider that gets the IAppTimer instance. Being an implementation detail, there’s no ITimerProvider dependency on the abstract interface.
The Tests
The previously-untestable user-defined functions now look like this:
Public Function TestUDF(ByVal SomeParameter As Double) As Boolean
On Error GoTo CleanFail
With AppContext.Current
Set .Target = .Caller.Offset(RowOffset:=1)
.Property("Test1") = 42
.Property("Test2") = 4.25 * SomeParameter
.Timer.ExecuteMacroAsync
End With
TestUDF = True
CleanExit:
Exit Function
CleanFail:
TestUDF = False
Resume CleanExit
Resume
End Function
The code isn’t very far off from the original, but now we can write a test that passes when a UDF invokes the Caller member; when the UDF is invoked from a worksheet cell, IAppContext.Caller returns the Range reference returned by Application.Caller; when the exact same code is invoked from a test, IAppContext.Caller returns a bogus/test cell reference.
Similarly, when a UDF invokes IAppTimer.ExecuteMacroAsync, a Win32 API call schedules the execution of a callback macro that itself invokes Application.OnTime to schedule the execution of a side-effecting macro that can consume the state and alter the target range and worksheet; when the exact same code is invoked from a test, IAppTimer.ExecuteMacroAsync simply notes that it was invoked, …and does nothing else.
This test passes when IAppTimer.ExecuteMacroAsync is invoked from a UDF, and would fail if the UDF didn’t invoke it:
'@TestMethod("Infrastructure")
Private Sub TestUDF_SchedulesMacro()
'inject the test factory:
Set AppContext.Factory = New TestContextFactory
'get the test context:
Dim Context As TestContext
Set Context = AppContext.Current
'test factory already stubbed the timer:
Dim StubTimer As TestTimer
Set StubTimer = AppContext.Current.Timer
'run the UDF:
Dim Result As Boolean
Result = Functions.TestUDF(0)
'Assert that the UDF has invoked IAppContext.ScheduleMacro once:
Const Expected As Long = 1
Assert.AreEqual Expected, StubTimer.ExecuteMacroAsyncInvokes, "IAppTimer.ExecuteMacroAsync was invoked " & StubTimer.ExecuteMacroAsyncInvokes & " times; expected " & Expected
End Sub
Cohesion
Ambient Context is a fantastic tool to address cross-cutting concerns and leverage global scope in a way that does not hinder testing. It’s also useful for storing state and dependencies that would otherwise be held in global scope, when passing that state and dependencies as normal parameters isn’t possible.
That makes it a somewhat dangerous pattern: one must keep in mind that the state is still global, and globals that don’t need to be global, should not be global. By defining an explicit interface for the context (like IAppContext), we not only end up with neat abstractions: we also make it harder for the context interface to grow new members and for the class to become an over-engineered Globals.bas module.
Interfaces shouldn’t be designed to change. In .NET the IDisposable interface only mandates a parameterless Dispose method; IEquatable is all about an Equals method. A factory interface shouldn’t need more than a carefully parameterized Create method that only takes arguments that can’t be dependencies of the factory instance: we want to avoid modifying existing interfaces as much as possible, and since none of us can really predict the future… the best way to do that is to keep interfaces as slim as possible. Cohesion is what we’re after: a module that is cohesive will feel like everything is exactly where it should be.
If the members of a module don’t feel like they’re a cohesive and complete group of closely related methods, there’s a greater chance that more members need to be added in the future – and you will want to avoid that. Of course the “and complete” part can mean a few growing pains, but in general naming things is a great way to avoid the pitfalls of treating the context as some “state bag” where we just lazily stuff state without thinking it through. In that sense AppContext is probably one of the worst possible names for this: perhaps a FunctionContext that only exposes the Caller member would be a cleaner approach?
In the real world, ambient context is for things like System.Threading.Thread.CurrentThread in .NET: it’s very specialized, with a very specific purpose, and we don’t see it very often. Authorization mechanisms might use it too.
In VBA-land, I’ve never once needed to implement it until I came upon this side-effecting UDF scenario needing unit tests; macros are definitely much simpler to refactor for testability!
Sync Project commands in the Code Explorer context menu.
VBA code being embedded in a host document might be very practical for certain aspects of both development and deployment, but let’s face it, it also makes using source control (e.g. git, SVN, mercurial, etc.) with VBA projects rather frustrating. As a developer, committing source code to a repository is usually a very simple task, because the code files live in the file system, and git can track changes and additions. With VBA, we commit the code that’s exported on the file system, the host document may contain different code, and merging remote changes implies exporting your code again, working out any merge conflicts with the exported code, then re-importing the merged changes into the host document.
Which wouldn’t be so bad… if the VBE had a nice way of exporting more than one single file at a time, and if importing files had an option to replace modules when they already exist… instead of importing the module with a “1” suffix as if that were something anyone ever needed to do!
Did You Know? The VBE’s “Import File…” command doesn’t make it very obvious, but it does support importing multiple files at once. Simply select multiple files when prompted for what file to import! Another little known feature of the VBE (one of the few Rubberduck hasn’t enhanced yet) is that its Project Explorer toolwindow is a drag-and-drop destination that can accept files you dragged from the Windows Explorer (⊞+E).
Rubberduck’s Export Project… command prompts for a folder, and then proceeds to export all modules there – overwriting any existing files in that folder. By default, the hotkey for that command is Ctrl+Shift+E, but it can be reconfigured to any key combination you like.
The context menu of Rubberduck’s Code Explorer toolwindow has a Sync Project sub-menu that offers two commands:
Update Components from Files…
Replace Contents from Files…
Rubberduck in general needs more documentation, but exactly what these “Sync Project” commands do is something that goes well beyond just using Rubberduck and they really deserve all the attention they can get, since they exist to facilitate an actual developmentworkflow that looks something like this:
This command prompts for source code files to import into your project.
If the project already contains a module with the same name as one of the imported files, the module is considered the same, and replaced with the imported version.
If the project does not already contain the imported modules, they’re simply added to the project.
If the project contains modules with different names than the imported files, these modules remain in the project.
Replace Contents
This command also prompts for source code files to import into your project, but the selected files will replace everything in the current project. Because this command is potentially destructive, a confirmation is required.
The entire project becomes the selected files.
If the project contained (non-document) modules before, they are removed before the import is performed.
We have a number of open issues (here, here, and here) about getting the “export project” command to take the @Folder annotations into account, and transpose the virtual folder hierarchy into an actual folder hierarchy on the file system, which would play nicely with version control and would help better organize a VBA repository.
In procedural code, a macro might be implemented in some Public Sub DoSomething procedure that proceeds to do whatever it is that it needs do, usually by dereferencing a number of library-defined objects and invoking their members in a top-to-bottom sequence of executable instructions. Clean, nicely written and well-modularized procedural code would have that be a small, high-abstraction public procedure at the top of some SomethingMacro standard module, with increasingly lower-abstraction private procedures underneath.
Looking only at scope names (the private procedures might be Function, and they would likely take parameters), the module for a MakeSalesReport macro might roughly look something like this:
Like “making coffee”, the phrase “make the sales report” is abstracting away quite a lot of smaller sub-steps.
Breaking down a problem into smaller and simpler steps and sub-steps is how we begin to achieve separation of concerns: maybe one of these sub-steps is going to require prompting the user for a filename – if that’s implemented in a separate PromptFileName function that’s only responsible for prompting the user for a filename, then it’s much easier to later (as needed) reuse that function by pulling it into its own, say, Files module, and making it Public.
If programming is a lot like writing a story, then procedures have to be the verbs we use to express the actions carried by our code. The smaller a procedure, the less it can do; the fewer things a procedure does, the easier it is to give it a name that accurately, precisely describes what it does.
Public Sub DoSomething()
'do stuff:
'...
'get the filename:
Dim FileName As String
FileName = ...
'do more stuff:
'...
End Sub
Any chunk of code that can be isolated inside a procedure scope and described with a comment that essentially says “this chunk of code reticulates splines” (whatever that is – maybe it’s “get the filename:”, or a much less subtle “======= GET FILENAME =======”), is a chunk of code that could be extracted into its own ReticulateSplines named procedure scope, and then doing this replaces a comment that says “this chunk of code reticulates splines” and the entire code block that goes with it, with a higher-abstraction single procedure call that plainly says ReticulateSplines: by properly naming the things we abstract away, we can make our code expressive and [for the most part] self-explanatory.
Option Explicit
Public Sub DoSomething()
DoStuff
Dim FileName As String
FileName = ...
DoMoreStuff FileName
End Sub
Private Sub DoStuff()
'...
End Sub
Private Sub DoMoreStuff(ByVal FileName As String)
'...
End Sub
And that’s glorious already.
With object-oriented programming (OOP), we get to further increase the abstraction level, such a Public Sub DoSomething macro procedure might belong to some Macros or EntryPoints standard module, painting an abstract broad-brush big picture… with all the spline-reticulating gory details in Private procedures of a separate class module.
Like procedures in procedural code, classes in OOP become another building block to tell our story: with class modules we get to use nouns: procedures do things, objects are things. So we could have a SomeMacro class that encapsulates everything “do something” needs to do, and when we need a DoSomethingElse macro we can implement it in its own dedicated class module too, leaving the Macros module (or EntryPoints, or whatever… just not Module1!) a high-abstraction, broad-brush picture of what’s going on.
This boils down to 1) create the dependencies of the macro class module we want to create; 2) create and initialize the “macro” object, and 3) invoke a Run method to, well, run the macro.
A standard module doing that, might look like this:
Option Explicit
Private Const ConnectionString As String = "..."
Public Sub DoSomething()
' create the dependencies...
Dim DbService As IDbService
Set DbService = SomeDbService.Create(ConnectionString)
' create the macro object, pass/inject the dependencies;
' we know SomeMacro needs a Worksheet and an IDbService
' because its Create factory method takes them as parameter:
With SomeMacro.Create(Sheet1, DbService)
.Run ' runs the macro
End With
End Sub
Public Sub DoSomethingElse()
'we could have another macro here...
'..if that other macro is in another class...
'...does it have a .Run method?
End Sub
This does effectively roughly demonstrate Dependency Injection and Inversion of Control in VBA (glossing over the required predeclared ID hidden attributes here), but in the context of this article, the point of interest is the .Run member call: if we make an object that encapsulates the notion of running a macro, it makes sense for that object to have a Run method. However if we don’t formalize this concept with an interface, we could have a SomeMacro.Run, then we could have AnotherMacro.Execute, and why not SomeOtherMacro.DoSomething: nothing is structuring things and telling the compiler and future maintainers “see this class is a macro and it has a method that runs it”, so while it’s nice that we’ve nicely cleaned up the Macros module by moving most of the code into class modules, it’s still chaos out there – unless there’s a way to get all macros to agree on exactly how we run them.
How do we tell the compiler “this class is a macro and it has a method that runs it”?
Interfaces and the Implements keyword, of course!
We can do this by adding a new class module (call it IMacro – I’m really not a prefix guy, but abstract interfaces in COM traditionally have that I prefix, and the tradition carried into C# and .NET, so here we are – if this were Java I would have just called it Macro; it’s all just conventions), and then adding a Run method with an empty body – this class shall remain abstract, and the implementation(s) shall be provided by other class modules:
'@ModuleDescription "Represents an executable macro."
'@Interface
Option Explicit
'@Description "Runs the macro."
Public Sub Run()
End Sub
The implementation(s) would be class modules with Implements IMacro and a Private Sub IMacro_Run procedure that invokes a Run procedure which… would break down into smaller, lower-abstraction private procedures underneath, and would delegate the more specialized work to more specialized objects (which would thus become that class’ dependencies). Sounds familiar?
Yep. You’re looking at your standard procedural macro, with the only difference being that instead of a standard module it’s now inside a class module that Implements IMacro.
Is this… a command pattern (macro in a class module)? Turns out, it pretty much actually is!
Of course, that’s not the whole story. But yes, it’s indeed a command pattern, however minimal – in design pattern abstraction terminology:
the caller is the Public Sub DoSomething macro procedure
the command is the IMacro interface
the concrete command is the SomeMacro class (implements IMacro)
the SomeDbService dependency would be a receiver, I think
What makes a “macro in a classs module” a command pattern, is the IMacro interface and how it abstracts the notion of “running a macro”. It represents the abstract concept of “something that can run”, and this right there, is the command pattern in a nutshell.
Let’s dig a little deeper though, because VBA can do much more than just macros, and commands are everywhere in software.
Divide & Conquer
Say we’re writing a user interface that can add, delete, and update records in a table. We might have a form featuring a ListBox control, and then CommandButton controls to create a new record, delete the selected one(s), and modify an existing one.
In a clean design without the command pattern, code might be written and organized with a “divide & conquer” attitude, and would look something like this (lower-abstraction details omitted, they’re not the point):
Option Explicit
'...
Public Property Get Model() As SomeModel
'gets an object holding the data needed for this form.
End Property
Private Sub CreateNewItem()
With New ItemEditorForm ' new form instance
.Show
If .Cancelled Then Exit Sub
AddToSource .Model ' implies the form has a Model As Something property.
End With
End Sub
Private Sub AddToSource(ByVal Thing As Something)
Model.AddThing Thing ' the Something class needs an AddThing method for this.
End Sub
Private Sub RemoveFromSource(ByVal Thing As Something)
Model.RemoveThing Thing ' the Something class needs a RemoveThing method for this.
End Sub
Private Sub DeleteSelectedItems()
Dim i As Long
For i = Me.ItemsBox.ListCount - 1 To 0 Step -1 ' assumes an ItemsBox listbox
If Me.ItemsBox.Selected(i) Then ' does not assume single-item selections
Dim Item As Something
' assumes a ListSource collection of Something objects
Set Item = ListSource(Me.ItemsBox.ListIndex)
If Not Item Is Nothing Then
RemoveFromSource Item ' <~ do this work at a lower abstraction level
End If
End If
Next
End Sub
Private Sub EditSelectedItem()
Dim Item As Something
Set Item = ListSource(Me.ItemsBox.ListIndex)
If Item Is Nothing Then Exit Sub
With New ItemEditorForm ' pop a modal with fields for an item...
Set .Model = Item ' <~ this item. (assumes a Model As Something property)
.Show
If .Cancelled Then Exit Sub
UpdateSourceItem .Model ' <~ do this work at a lower abstraction level
End With
End Sub
Private Sub CreateButton_Click()
CreateNewItem ' <~ do this work at a lower abstraction level
End Sub
Private Sub DeleteButton_Click()
DeleteSelectedItems ' <~ do this work at a lower abstraction level
End Sub
Private Sub EditButton_Click()
EditSelectedItem ' <~ do this work at a lower abstraction level
End Sub
'...
By factoring each button action into its own dedicated procedure, we get to name things and clearly split things up by functionality. The job of a Click handler becomes to fork execution elsewhere, so they [often] become simple one-liners invoking a private method, painting a broad-brush picture of what’s going on.
We could just as well implement the functionality in the body of the Click handler, but I personally find extracting these private methods worthwhile, because they make it easier to restructure things later (you can cut/move the entire scope), versus leaving that code in event handlers where the refactoring is more tedious. Event handlers are entry points in a way, enough so that having them at a high abstraction level feels exactly right for me.
Now what if we wanted the EditButton to only be enabled when only one item is selected, and then make the DeleteButton only enabled when at least one item is selected? We would have to start handling the ItemsBox.Change event, and would need additional code that might look like this:
Private Sub SetButtonsEnabledState()
Me.EditButton.Enabled = (Model.SelectedItems.Count = 1)
Me.DeleteButton.Enabled = (Model.SelectedItems.Count > 0)
'...
End Sub
Private Sub ItemsBox_Change()
SetModelSelectedItems
SetButtonsEnabledState
End Sub
Imagine a form with many more controls – each with their own “is enabled” rules and a Change event handler procedure: boilerplate… boilerplate code everywhere!
Each command button has its own associated actions implemented in its own set of procedures, and that creates a lot of noise and reduces the signal when we’re reading the code, and that’s a clear sign the abstraction level needs to go up a bit.
Abstraction Levels Think of the steps involved in making a cup of coffee, in maybe 3-5 steps. Think of a descriptive verb for each step, then think of how each step could be broken down into another 3-5 steps, and then use descriptive names for these steps, too. The names at the top level are necessarily going to be more abstract than those in the lower level(s): that’s what abstraction levels refers to. Now imagine doing all that in one giant procedure scope and you can see the benefits of balancing abstraction and indirection in programming 🙂
Moving that boilerplate to Public procedures in standard modules would “work” to clean up the form module… but then it would also pretty much defeat the purpose of encapsulating things into objects… and then when (not if) one such procedure needs any state, then that state soon becomes global state, and that is absolutely not something we want to have to resort to.
Command & Conquer
Using the command pattern (even without MVVM command bindings), a CreateButton_Click handler would still be responsible for kicking the “create a new item” logic into action… but now that logic would be living in some ICommand implementation, encapsulating its dependencies and state (and thus moving these outside of the form’s code-behind but not into global scope now).
'@Folder MVVM.Infrastructure.Abstract
'@ModuleDescription "An object that represents an executable command."
'@Interface
'@Exposed
Option Explicit
'@Description "Returns True if the command is enabled given the provided binding context (ViewModel)."
Public Function CanExecute(ByVal Context As Object) As Boolean
End Function
'@Description "Executes the command given the provided binding context (ViewModel)."
Public Sub Execute(ByVal Context As Object)
End Sub
'@Description "Gets a user-friendly description of the command."
Public Property Get Description() As String
End Property
This makes a command as an abstraction that has:
A user-friendly description of what the command does.
A function that takes a context object and returns a Boolean value that indicates whether the command can currently be executed.
An Execute procedure that takes a context object and, well, executes the command.
The mysterious Context parameter is an object that encapsulates the state, the data we’re working with. In MVVM that would be the ViewModel instance.
MVVM command bindings use the Description property to set the ControlToolTip string of a binding’s targetCommandButton object, and automatically invokes the CanExecute method as property bindings update, which automatically enables or disables the bound command button control: the command pattern works very, very well with Model-View-ViewModel, but nothing says we cannot use the command pattern without it.
So let’s strip the interface of its Description property, leaving only the CanExecute and Execute methods:
'@Folder CommandPattern.Example
'@ModuleDescription "An object that represents an executable command."
'@Interface
'@Exposed
Option Explicit
'@Description "Returns True if the command is enabled given the provided context."
Public Function CanExecute(ByVal Context As Object) As Boolean
End Function
'@Description "Executes the command given the provided context."
Public Sub Execute(ByVal Context As Object)
End Sub
We’re still going to need a Click handler in the code-behind for each CommandButton on a form, but now that we have an ICommand abstraction to code against, we can already go back to the Divide & Conquer form’s code-behind and watch it melt:
Private CreateNewItem As ICommand
Private DeletedSelectedItems As ICommand
Private EditSelectedItem As ICommand
Public Property Get Model() As Object
'gets an object holding the data needed for this form
End Property
Private Sub CreateButton_Click()
CreateNewItem.Execute Me.Model
End Sub
Private Sub DeleteButton_Click()
DeleteSelectedItems.Execute Me.Model
End Sub
Private Sub EditButton_Click()
EditSelectedItem.Execute Me.Model
End Sub
That of course is again just simplified illustrative code, but the lower-abstraction implementation details that were omitted for brevity in the “divide & conquer” code no longer need to find a place to call home, and no longer even need to be omitted either: that lower-abstraction code is simply gone from the code-behind now, and lives in a handful of distinct objects that implement the ICommand interface, such that the only thing a button’s Click handler needs to do now is to invoke a high-abstraction method that does whatever it needs to do.
At a glance, such a one-liner CreateNewItem.Execute instruction looks very similar to another one-liner CreateNewItem instruction (both involve a procedure call against an object – but only one of them is a command); the difference is that now the form is [blissfully] unaware of how that activity is going to happen, and a maintainer looking for the code that creates a new item will find it in a CreateNewItemCommand class, instead of somewhere in the middle of other specialized procedure scopes all in the same module.
Embracing Changes
Code changes, code evolves, it’s inevitable: code lives. When we code against abstractions, we reduce the code’s resistance to change. You want your code to embrace changes, you want it to welcome changes and extensions.
By coding against an ICommand interface, the only thing we commit to is that clicking a button will do something; we don’t know what and we don’t even need to care, and that’s what not resisting change means: we aren’t saying “run procedure X in module Y” anymore, we’re saying “run X implemented by any class whatsoever“. The actual code that runs the command is bound at run-time and doesn’t even need to exist for the code to compile, and the form is still fully-functional given no-op stub “commands” – we just need to get more abstract about what “to be functional” means for a form (meaning, if we click a button and ICommand.Execute is invoked, then we’re good – that’s all we need the form to do here).
The hypothetical example code above implies a separate CreateItemCommand class; it might look something like this:
Option Explicit
Implements ICommand
Private Function ICommand_CanExecute(ByVal Context As Object) As Boolean
ICommand_CanExecute = True
End Function
Private Sub ICommand_Execute(ByVal Context As Object)
With New ItemEditorForm
.Show
If .Cancelled Then Exit Sub
AddToSource .Model, Context
End With
End Sub
Private Sub AddToSource(ByVal Thing As Something, ByVal Context As Object)
Context.AddThing Thing
End Sub
Note that this is again really just moving private methods from one place into their own class, so AddToSource would be the same code as before, only now the “source” collection that needs an item added to, would live in the Context object, which we’re accessing late-bound here for simplicity’s sake, but a command implementation that works with a particular specific type of Context object should validate that, and cast the parameter into a local variable declared with the appropriate type, so as to avoid such unnecessary late binding, like this:
Private Sub DoSomething(ByVal Context As Object)
Debug.Assert TypeOf Context Is Class1
Dim LocalContext As Class1
Set LocalContext = Context '<~ type mismatch here if the assert fails
'carry on using LocalContext with early-bound member calls
End Sub
By moving the implementation out of the button’s Click handler, we make it much easier to later repurpose that button, or to make a future button elsewhere that invokes the same command. The form module doesn’t need to know about any concrete implementation of the ICommand interface: a button can be wired-up to any command, swapping SomeCommand for a SomeOtherCommand implementation is all that’s needed.
One Step Further
We’ve seen how to pull functionality from a form’s code-behind and refactor it into specialized command objects that can be invoked from a button’s Click handler. The nicest thing about such commands, is that they are full-fledged objects, which means they can be passed around as parameters – and Model-View-ViewModel (MVVM) leverages that.
In the MVVM object model, you have a top-level AppContext object that exposes an ICommandManager object: this manager is responsible for holding a reference to all command bindings in your MVVM application, and there’s an IBindingManager that notifies it whenever a property binding updates in a way that may require commands’ CanExecute method to be evaluated.
When coding against the MVVM object model, you no longer wire-up event handlers: the MVVM infrastructure automatically does it for you – so the only code that remains (that actually does anything) in a form’s code-behind, is code that wires up form controls to property and command bindings – the rest is just implementations for IView and ICancellable interfaces (as applicable), and then a factory method can initialize a bunch of properties (or the properties can be Set from outside the module, but a Create factory method works very well with UserForm classes for property injection):
Option Explicit
Implements IView
Implements ICancellable
Private Type TState
Context As MVVM.IAppContext
ViewModel As ExampleViewModel '<~ any class implementing INotifyPropertyChanged
IsCancelled As Boolean
CreateNewItem As ICommand
DeletedSelectedItems As ICommand
EditSelectedItem As ICommand
End Type
Private This As TState
'...properties...
Public Property Get ViewModel() As ExampleViewModel
Set ViewModel = This.ViewModel
End Property
Private Sub InitializeView()
With This.Context.Commands
.BindCommand ViewModel, Me.CreateButton, ViewModel.CreateNewItem
.BindCommand ViewModel, Me.DeleteButton, ViewModel.DeleteSelectedItems
.BindCommand ViewModel, Me.EditButton, ViewModel.EditSelectedItem
.BindCommand ViewModel, Me.CancelButton, CancelCommand.Create(Me)
End With
End Sub
'...interface implementations...
The UI controls are still referred to as Me.CreateButton, Me.DeleteButton, and Me.EditButton (added Me.CancelButton for good measure), but now instead of handling their Click event we bind them to ICommand objects – whose references we conveniently expose as Property Get members of our ViewModel, but we can also bind a command that we create inline, like this CancelCommand instance. Shame the QueryClose event isn’t exposed, because then binding a CancelCommand to a UserForm would be all you’d need to do for it to automagically properly close/cancel a dialog.
Note that the form doesn’t even need to know what specific ICommand implementations it’s given to work with, at all: here the form is coupled with the CancelCommand, but all other commands (create, delete, edit) are binding to public ICommand properties that live on the ViewModel object.
Full Circle: EventCommand (MVVM)
Not all commands are created equal: a command like CancelCommand is generic enough that it can work with any ICancellable object, and an AcceptCommand can work with any implementation of the IView interface. On the other hand, something feels wrong about systematically implementing any & all commands in their own classes.
Having each command neatly factored into its own class module is a great way to implement complex commands, but can be overkill when things are relatively trivial – very often the ViewModel class already has access to every object a command needs, and having a way to make the ViewModel itself implement the command would solve this.
I’m going to introduce an EventCommand class into the MVVM infrastructure code, to do exactly this:
'@Folder MVVM.Common.Commands
'@ModuleDescription "A command that allows the ViewModel to supply the implementation."
'@PredeclaredId
'@Exposed
Option Explicit
Implements ICommand
Private Type TState
Description As String
End Type
Private This As TState
Public Event OnCanExecute(ByVal Context As Object, ByRef outResult As Boolean)
Public Event OnExecute(ByVal Context As Object)
'@Description "Creates a new instance of this ICommand class. Set the returned reference to a WithEvents variable."
Public Function Create(ByVal Description As String) As ICommand
Dim Result As EventCommand
Set Result = New EventCommand
Result.Description = Description
Set Create = Result
End Function
'@Description "Gets/sets the command's Description."
Public Property Get Description() As String
Description = This.Description
End Property
Friend Property Let Description(ByVal RHS As String)
This.Description = RHS
End Property
Private Function ICommand_CanExecute(ByVal Context As Object) As Boolean
Dim outResult As Boolean
outResult = True
RaiseEvent OnCanExecute(Context, outResult)
ICommand_CanExecute = outResult
End Function
Private Property Get ICommand_Description() As String
ICommand_Description = This.Description
End Property
Private Sub ICommand_Execute(ByVal Context As Object)
RaiseEvent OnExecute(Context)
End Sub
In VBA we can’t pass functions around like we can with delegates in C#, but events are a nice language feature we can still leverage for this purpose. Code like this could be in any ViewModel class:
Private WithEvents PseudoDelegateCommand As EventCommand
'...
Private Sub Class_Initialize()
Set PseudoDelegateCommand = EventCommand.Create("Full circle!")
End Sub
'...
Private Sub PseudoDelegateCommand_OnCanExecute(ByVal Context As Object, outResult As Boolean)
'supply the ICommand.CanExecute implementation here.
'assign outResult to False to disable the command (it's True by default).
'in principle, the Context *is* the ViewModel instance, so this assertion should hold:
Debug.Assert Me Is Context
'it also means the Context parameter should probably be ignored.
End Sub
Private Sub PseudoDelegateCommand_OnExecute(ByVal Context As Object)
'supply the ICommand.Execute implementation here.
'in principle, the Context *is* the ViewModel instance, so this assertion should hold:
Debug.Assert Me Is Context
'it also means the Context parameter should probably be ignored.
'EventCommand is useful for commands that are specific to a particular ViewModel,
'and don't really need to have their implementation extracted into their own class.
End Sub
And now we’ve gone full circle and essentially moved the Click handlers out of the View …and into the ViewModel – except these aren’t Click handlers now, although they will run when a user clicks the associated button (mind-boggling, right?): we’re essentially looking at callbacks here, invoked from within the MVVM infrastructure in response to control events… and/or INotifyPropertyChanged notifications from the ViewModel.
From a testability standpoint, it’s important to understand the implications: if you intend to have your ViewModel under a thorough suite of unit tests, then an EventCommand becomes somewhat of a liability. The OnExecute handler (or OnCanExecute, for that matter) shouldn’t require dependencies that the ViewModel doesn’t already have, so that tests can property-inject stub dependencies. In other words, unless the ViewModel already depends on an abstraction to access, say, a database connection or the file system, then the handlers of an EventCommand in that class shouldn’t connect to a database or access the file system.
You’re in command
Whether it’s for a workbook with many simple (-ish) macros, or for a full-fledged MVP, MVC, or MVVM application, implementing the command pattern lets you move the code that contains your actual functionality wherever it makes the most sense to have it. Unless you’re writing a Smart UI, that place is pretty much never the code-behind of the View module. By implementing an ICommand interface directly, you can move all that code from the UI to a command class whose sole purpose is to provide that particular piece of functionality.
Using an EventCommand with MVVM, you can even move that code from the UI to literally anywhere you want, as long as that is a class module (only class modules can have a WithEvents instance variable). It’s not uncommon to see a ViewModel class include somewhat high-abstraction code that provides commands’ implementations.
See and follow github.com/rubberduck-vba/MVVM for the Model-View-ViewModel infrastructure code that makes command bindings a thing in VBA, as well as examples (including a Smart UI!) and additional documentation.
Using a WithEvents variable to handle the MSForms.Control events of, say, a TextBox control has the irritating tendency to throw a rather puzzling run-time error 459 “Object or class does not support the set of events”. To be honest, I had completely forgotten about this when I started working on this MVVM framework. I had even posted an answer on Stack Overflow and my learning-it-the-hard-way is immortalized on that page.
…there’s a bit of COM hackery going on behind the scenes; there’s enough smokes & mirrors for VBA to successfully compile the above, but, basically, you’re looking at a glitch in The Matrix (Rubberduck’s resolver has similar “nope” issues with MSForms controls): there isn’t any obvious way to get VBA to bind a dynamic control object to its MSForms.Control events.
-Mathieu Guindon, Apr 18 ’19
What I hadn’t noticed until today, was that another user had posted an answer to that question a few hours later that day – and that answer ultimately leads to the groundbreaking manual wiring-up of what VBA normally does automagically under the hood when we declare a WithEvents variable.
pUnk’d
The code I’m about to share is heavily based on the work shared on Stack Overflow by user Evr, and uses the ConnectToConnectionPoint Win32 API that, it must be mentioned, comes with a caveat:
This function is available through Windows XP and Windows Server 2003. It might be altered or unavailable in subsequent versions of Windows.
Regardless, it works (for now anyway, …if we lose Mac support for this specific capability).
Rubberduck uses similar connection points to handle a number of VBE events that aren’t otherwise exposed, so I knew this was going to work one way or another. The idea is to pass an IUnknown pointer to an object that exposes members with very specific VB_UserMemId attribute values, and have accordingly very specific member signatures.
This post lists a bunch of such attributes – however since there aren’t any problems with binding regular TextBox and CommandButton events (these do work with simple WithEvents event providers), I’m only interested in these:
Event
VB_UserMemId
AfterUpdate
-2147384832
BeforeUpdate
-2147384831
Enter
-2147384830
Exit
-2147384829
The VB_UserMemId attribute values for each of the MSForms.Control events.
This is going to be a little bit lower-level than usual, but every VBA user class has an IUnknown pointer, So we can use any class module that has the members with the appropriate VB_UserMemId attribute values, and pass that as the pUnk pointer argument.
So, here’s the punk in question, exactly as I currently have it:
VERSION 1.0 CLASS
BEGIN
MultiUse = -1 'True
END
Attribute VB_Name = "ControlEventsPunk"
Attribute VB_GlobalNameSpace = False
Attribute VB_Creatable = False
Attribute VB_PredeclaredId = False
Attribute VB_Exposed = False
Attribute VB_Description = "Provides an event sink to relay MSForms.Control events."
'@Folder MVVM.Infrastructure.Win32
'@ModuleDescription "Provides an event sink to relay MSForms.Control events."
'based on https://stackoverflow.com/a/51936950
Option Explicit
Implements IControlEvents
Private Type GUID
Data1 As Long
Data2 As Integer
Data3 As Integer
Data4(0 To 7) As Byte
End Type
'[This function is available through Windows XP and Windows Server 2003. It might be altered or unavailable in subsequent versions of Windows.]
'https://docs.microsoft.com/en-us/windows/win32/api/shlwapi/nf-shlwapi-connecttoconnectionpoint
#If VBA7 Then
Private Declare PtrSafe Function ConnectToConnectionPoint Lib "shlwapi" Alias "#168" (ByVal Punk As stdole.IUnknown, ByRef riidEvent As GUID, ByVal fConnect As Long, ByVal PunkTarget As stdole.IUnknown, ByRef pdwCookie As Long, Optional ByVal ppcpOut As LongPtr) As Long
#Else
Private Declare Function ConnectToConnectionPoint Lib "shlwapi" Alias "#168" (ByVal punk As stdole.IUnknown, ByRef riidEvent As GUID, ByVal fConnect As Long, ByVal punkTarget As stdole.IUnknown, ByRef pdwCookie As Long, Optional ByVal ppcpOut As Long) As Long
#End If
Private Type TState
RefIID As GUID 'The IID of the interface on the connection point container whose connection point object is being requested.
Connected As Boolean
PunkTarget As Object
Cookie As Long
Handlers As Collection
End Type
'from https://stackoverflow.com/a/61893857 (same user as #51936950!)
Private Const ExitEventID As Long = -2147384829
Private Const EnterEventID As Long = -2147384830
Private Const BeforeUpdateEventID As Long = -2147384831
Private Const AfterUpdateEventID As Long = -2147384832
Private This As TState
'@Description "Gets/sets the target MSForms.Control reference."
Public Property Get Target() As Object
Attribute Target.VB_Description = "Gets/sets the target MSForms.Control reference."
Set Target = This.PunkTarget
End Property
Public Property Set Target(ByVal RHS As Object)
Set This.PunkTarget = RHS
End Property
'@Description "Registers the listener."
Public Function Connect() As Boolean
Attribute Connect.VB_Description = "Registers the listener."
GuardClauses.GuardNullReference This.PunkTarget, TypeName(Me), "Target is not set."
ConnectToConnectionPoint Me, This.RefIID, True, This.PunkTarget, This.Cookie, 0&
This.Connected = This.Cookie <> 0
Connect = This.Connected
End Function
'@Description "De-registers the listener."
Public Function Disconnect() As Boolean
Attribute Connect.VB_Description = "De-registers the listener."
If Not This.Connected Then Exit Function
ConnectToConnectionPoint Me, This.RefIID, False, This.PunkTarget, This.Cookie, 0&
This.Connected = False
Disconnect = True
End Function
'@Description "A callback that handles MSForms.Control.AfterUpdate events for the registered target control."
Public Sub OnAfterUpdate()
Attribute OnAfterUpdate.VB_UserMemId = -2147384832
Attribute OnAfterUpdate.VB_Description = "A callback that handles MSForms.Control.AfterUpdate events for the registered target control."
Dim Handler As IHandleControlEvents
For Each Handler In This.Handlers
Handler.HandleAfterUpdate
Next
End Sub
'@Description "A callback that handles MSForms.Control.BeforeUpdate events for the registered target control."
Public Sub OnBeforeUpdate(ByVal Cancel As MSForms.ReturnBoolean)
Attribute OnBeforeUpdate.VB_UserMemId = -2147384831
Attribute OnBeforeUpdate.VB_Description = "A callback that handles MSForms.Control.BeforeUpdate events for the registered target control."
Dim Handler As IHandleControlEvents
For Each Handler In This.Handlers
Handler.HandleBeforeUpdate Cancel
Next
End Sub
'@Description "A callback that handles MSForms.Control.Exit events for the registered target control."
Public Sub OnExit(ByVal Cancel As MSForms.ReturnBoolean)
Attribute OnExit.VB_UserMemId = -2147384829
Attribute OnExit.VB_Description = "A callback that handles MSForms.Control.Exit events for the registered target control."
Dim Handler As IHandleControlEvents
For Each Handler In This.Handlers
Handler.HandleExit Cancel
Next
End Sub
'@Description "A callback that handles MSForms.Control.Enter events for the registered target control."
Public Sub OnEnter()
Attribute OnEnter.VB_UserMemId = -2147384830
Attribute OnEnter.VB_Description = "A callback that handles MSForms.Control.Enter events for the registered target control."
Dim Handler As IHandleControlEvents
For Each Handler In This.Handlers
Handler.HandleEnter
Next
End Sub
'@Description "Registers the specified object to handle the relayed control events."
Public Sub RegisterHandler(ByVal Handler As IHandleControlEvents)
Attribute RegisterHandler.VB_Description = "Registers the specified object to handle the relayed control events."
This.Handlers.Add Handler
End Sub
Private Sub Class_Initialize()
Set This.Handlers = New Collection
This.RefIID.Data1 = &H20400
This.RefIID.Data4(0) = &HC0
This.RefIID.Data4(7) = &H46
End Sub
Private Sub IControlEvents_OnAfterUpdate()
OnAfterUpdate
End Sub
Private Sub IControlEvents_OnBeforeUpdate(ByVal Cancel As MSForms.IReturnBoolean)
OnBeforeUpdate Cancel
End Sub
Private Sub IControlEvents_OnEnter()
OnEnter
End Sub
Private Sub IControlEvents_OnExit(ByVal Cancel As MSForms.IReturnBoolean)
OnExit Cancel
End Sub
Private Sub IControlEvents_RegisterHandler(ByVal Handler As IHandleControlEvents)
RegisterHandler Handler
End Sub
Let’s ignore the IControlEvents interface for now. The class has a Target – that’ll be our TextBox control instance. So we set the Target, and then we can invoke Connect, and when we’re done we can invoke Disconnect to explicitly undo the wiring-up.
Then we have an OnEnter method with VB_UserMemId = -2147384830, which makes it an event handler procedure for MSForms.Control.Enter. The name of the procedure isn’t relevant, but it’s important that the procedure is parameterless.
Similarly, the name of the OnExit procedure has no importance, but it must have a single ByVal Cancel As MSForms.ReturnBoolean parameter (only ByVal and the data type matter). For events that have more than one parameter, the order is also important.
In theory that’s all we need: we could go on and handle Control.Exit in this OnExit procedure, and call it a day. In fact you can probably do that right away – however I need another step for my purposes, because I’m going to need my PropertyBindingBase class to propagate these events “up” to, say, some TextBoxPropertyBinding class that can implement some TextBox-specific behavior for the Control events.
Propagating Events
I had already a working pattern for my INotifyPropertyChange requirements to propagate property changes across objects, and the pattern is applicable here too. See, I could have declared a Public Event Exit(ByRef Cancel As MSForms.ReturnBoolean) on the ControlEventsPunk class, and then I could have used a WithEvents variable to handle them – and that would have worked too. Except I don’t want to use events here, because events work well as implementation details… but they can’t be exposed on an interface, which makes them actually more complicated to work with.
There are two interfaces: one that defines the “events” and exposes a method to register “handlers”, and the other mandates the presence of a callback for each “event”. For INotifyPropertyChange the handler interface was named IHandlePropertyChange, so I went with IControlEvents and IHandleControlEvents.
So, the “provider” interface looks like this:
'@Folder MVVM.Infrastructure.Bindings.Abstract
'@ModuleDescription "Provides the infrastructure to relay MSForms.Control events."
Option Explicit
Public Sub RegisterHandler(ByVal Handler As IHandleControlEvents)
End Sub
Public Sub OnEnter()
End Sub
Public Sub OnExit(ByVal Cancel As MSForms.ReturnBoolean)
End Sub
Public Sub OnAfterUpdate()
End Sub
Public Sub OnBeforeUpdate(ByVal Cancel As MSForms.ReturnBoolean)
End Sub
And then the “handler” interface looks like this:
'@Folder MVVM.Infrastructure.Bindings.Abstract
'@ModuleDescription "An object that can be registered as a handler for IControlEvents callbacks."
Option Explicit
Public Sub HandleEnter()
End Sub
Public Sub HandleExit(ByVal Cancel As MSForms.ReturnBoolean)
End Sub
Public Sub HandleAfterUpdate()
End Sub
Public Sub HandleBeforeUpdate(ByVal Cancel As MSForms.ReturnBoolean)
End Sub
So, looking back at the ControlEventsPunk class, we find that the implementation for RegisterHandler consists in adding the provided Handler object to an encapsulated Collection that holds all the registered handlers; when we “handle” a control event, we iterate all registered handlers and invoke them all in a sequence. When an event has a Cancel parameter, the last handler that ran gets the final say on whether the parameter should be True or False, and each handler receives the Cancel value that was set by the previous handler than ran.
This is a slightly different paradigm than your regular VBA/VB6 auto-wired events, where one event only ever has one handler: now these work more like the multicast delegates that events are in .NET, with an “invocation list” and the ability to add/remove (although, I haven’t implemented the removal) handlers dynamically at run-time – except the “handlers” are full-fledged VBA objects here, rather than .NET delegates.
Whenever the MVVM infrastructure needs to propagate events, I use this pattern instead. This was my first time actually implementing an Observer Pattern, and hadn’t even realized! (thanks Max!) – that isn’t a pattern you see often in event-capable languages, but I can definitely see this proven, solid abstraction (Java developers would probably be rather familiar with that one) become my new favorite go-to pattern to expose events on an interface in VBA… But there’s probably a reason the first time I come across a situation where that pattern is really handy (and actually needed, for testability), is when I’m writing framework-level (i.e. an API intended to be used by code that isn’t written yet) code that’s very much as deep into the OOP rabbit hole as I’ve ever been in VBA (or any other language for that matter)… and there’s still no rock bottom in sight.
In any case, now that we have a way to handle and propagate control events, we can have MVVM property bindings that can format TextBox.Text on exit, i.e. we can have a ViewModel that knows SomeProperty has a value of 25.59, and the Text property of the bound textbox control can say $25.59 just by specifying a FormatString (like “Currency”, for example) when we create the binding.
For the next post in this series I think we’re ready to deep-dive into the actual binding mechanics, and I’ll have the updated MVVM infrastructure code on GitHub by then.
Rubberduck News 