Declaring and Using Variables in VBA

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()
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()
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 qualify Range and Cells member calls with a proper Worksheet object.

For example before might look like this:

Sub Macro1
    Range("A10") = 42
    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
    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.

Lightweight MVVM in VBA

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.

The accompanying code for this article is in the Rubberduck Examples repository.

Property Bindings

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
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.


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.


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:

Private Sub ConfigureBindings(ByVal Model As INotifyPropertyChanged)

    Const EnabledProperty As String = "Enabled"
    Const ListProperty As String = "List"
    This.Bindings.Add BindTextBox(Me.BillToNameBox, "BillToName", This.OrderModel)
    This.Bindings.Add BindTextBox(Me.BillToAddressLine1, "BillToLine1", This.OrderModel)
    This.Bindings.Add BindTextBox(Me.BillToAddressLine2, "BillToLine2", This.OrderModel)
    This.Bindings.Add BindTextBox(Me.BillToAddressLine3, "BillToLine3", This.OrderModel)
    This.Bindings.Add BindTextBox(Me.BillToEmailBox, "EmailAddress", This.OrderModel)
    This.Bindings.Add BindCheckBox(Me.BillToContributorBox, "IsContributor", This.OrderModel)
    This.Bindings.Add BindCheckBox(Me.ShipToSameBox, "ShipToBillingAddress", This.OrderModel)
    This.Bindings.Add BindTextBox(Me.ShipToNameBox, "ShipToName", This.OrderModel)
    This.Bindings.Add BindTextBox(Me.ShipToAddressLine1, "ShipToLine1", This.OrderModel)
    This.Bindings.Add BindTextBox(Me.ShipToAddressLine2, "ShipToLine2", This.OrderModel)
    This.Bindings.Add BindTextBox(Me.ShipToAddressLine3, "ShipToLine3", This.OrderModel)
    This.Bindings.Add BindProperty(Me.ShipToAddressLabel, EnabledProperty, "ShipToBillingAddress", This.OrderModel, InvertBoolean:=True)
    This.Bindings.Add BindProperty(Me.ShipToNameLabel, EnabledProperty, "ShipToBillingAddress", This.OrderModel, InvertBoolean:=True)
    This.Bindings.Add BindProperty(Me.ShipToNameBox, EnabledProperty, "ShipToBillingAddress", This.OrderModel, InvertBoolean:=True)
    This.Bindings.Add BindProperty(Me.ShipToAddressLine1, EnabledProperty, "ShipToBillingAddress", This.OrderModel, InvertBoolean:=True)
    This.Bindings.Add BindProperty(Me.ShipToAddressLine2, EnabledProperty, "ShipToBillingAddress", This.OrderModel, InvertBoolean:=True)
    This.Bindings.Add BindProperty(Me.ShipToAddressLine3, EnabledProperty, "ShipToBillingAddress", This.OrderModel, InvertBoolean:=True)
    This.Bindings.Add BindProperty(Me.ItemSkuSelectBox, ListProperty, "Value", InventorySheet.Table.ListColumns("SKU").DataBodyRange)
    This.Bindings.Add BindComboBox(Me.ItemSkuSelectBox, "SKU", This.OrderModel.NewLineItem)
    This.Bindings.Add BindTextBox(Me.ItemQuantityBox, "Quantity", This.OrderModel.NewLineItem)
    This.Bindings.Add BindTextBox(Me.ItemPriceBox, "Price", This.OrderModel.NewLineItem)
    This.Bindings.Add BindProperty(Me.LineItemsList, ListProperty, "LineItems", This.OrderModel)

End Sub

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.


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.


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.

Rubberduck Swag Shop is Open!

Back in the summer of 2017, we ran a rather successful GoFundMe campaign to collect funds to get some swag made, and shipped to the project’s contributors. It was great to see all that support pouring in, but it was bittersweet… for I would have loved to have been able to materially thank back our supporters. Alas, making custom printed gear quickly gets expensive, and crazy shipping fees essentially doubled it up.

So this time around I decided to bite the bullet and make a more significant initial investment, and actually start an online shop and keep an inventory in stock that can be shipped to supporters whenever, without needing to wait for prints, or paying too much for too few items being made, severely limiting profitability and driving prices up.

I guess I’ll know soon enough if I still have my old merchandise planner’s touch! If the inventory turns too fast, I’ll miss opportunities and may have to delay shipments to replenish. But if the markup is too high and stock isn’t moving quickly enough, I’ll probably need to take markdowns.

I’ll be writing an article about the particular workbook I’m using to track all that next – I want a bit of VBA code to simplify tracking sales and invoicing.. but for now I’m just thrilled to start shipping all the goodies!

I’m very happy with how everything turned out. That beautiful yellow ducky mug will probably only ever be printed in small batches, for they are quite expensive and I’m unfortunately not going to be able to have a better price for them unless I make them in very large batches. Everything else (even t-shirts, to a degree) has the potential to eventually get to a very decent price point – the goal is obviously to collect some funds to help finance the costs of hosting and securing the blog and website, but I want this inventory to move and don’t mind taking a lower margin if there’s enough volume to drive enough sales to just keep the shop afloat – time will tell!

Rubberduck Swag Shop on Ko-fi

Rubberduck 3.0: February Update

I’ve embarked on a journey to take Rubberduck to the next major version, making it the add-in we’ve always wanted to build. These monthly updates provide a sneak peek at what’s coming, and how it’s coming to be.

Quick catch-up

  • Rubberduck 3.0 will run a LSP server process in the background
  • A separate process will host a local SQLite database
  • Telemetry will be opt-in, fully configurable, and transparent
  • January update

Several quite important low-level changes since last time: we’re now looking at named pipes rather than sockets for the JSON-RPC communications between the editor and the language server, and I’m now using Microsoft’s StreamJsonRpc library for this. Named pipes are inherently local, so they’re less of a concern than sockets, and they don’t seem to trip up Windows Defender, so we’ll take it!

I spent the better part of last month tidying up and documenting the code off the language server protocol (LSP) specifications, moving things around and splitting up responsibilities, writing abstractions that will be shared by all server processes: LSP and SQLite, but also a separate/dedicated server process for telemetry, so even constant writes couldn’t interfere much with LSP server activities, or with the add-in client.

At the time of this writing, I’m still somewhat struggling with the RPC communications, but that won’t remain stuck for too long – the plan is to merge a rather large structural PR and the whole RPC infra by the end of this week.

The Rubberduck.Server.LocalDb console at startup. This process will normally run in the background, hidden.

I’ve taken a number of important decisions about the project in the last few weeks.

GitHub Repository Issues

Since the project’s beginning, Rubberduck was pretty much ad-hoc development. I remember in the first few days after creating the GitHub repository, going on an issue-creation spree to write down everything I could dream the thing could do. A lot of it was implemented, but the oldest open issues in the repository are from 2014, 2015:

2,857 issues closed is quite something. 958 open feels daunting though. Using issues as a backlog might not have been the best of ideas…

I’m not going through nearly a thousand issues to sort out what’s already implemented/fixed, what’s unrealistic after all, what’s a good idea that got buried under a million others, etc. Implementing LSP isn’t magically going to clean this up, and when 3.0 releases we’re not going to be maintaining two distinct, massive code bases: one of them isn’t going to make it, and it’s sadly going to have to be the one with 1.7K stars and 284 forks and 97 watchers. I can hardly express how I feel about these numbers, let alone those:

As of 2023/02/14, release tag (build has been downloaded 27,320 times… that’s crazy!

The repository isn’t going anywhere though – it’s just that at some point in the [somewhat-near] future, it’s going to be made read-only and essentially archived, and the Rubberduck3 repository will become Rubberduck’s new home on GitHub (you’ll still find it under the rubberduck-vba organization).

Rubberduck is still accepting pull requests for v2.x and will continue to do so until further notice.

Methodology Upgrade

If building Rubberduck up to v2.5 was pretty much ad-hoc (and that’s fine!), I don’t think the same strategy would work with v3.0; we can’t just go and create a thousand issues to churn through, or pick a feature to implement because it looks like it’s going to be fun to do. Rubberduck 3.0 is still an embryo at this point, and while all the DNA is there and we know exactly (at least in large fuzzy outlines) what we want this add-in to do, this time things need to happen in order, for technical reasons mostly, but also for project management.

By adopting a different development methodology, we’re going to better control the backlog and project progression. We can better track what’s in progress and determine what the next logical steps should be.

Instead of making a ton of issues, we’ll be drafting them, sizing them, prioritizing them, refining them until they’re small enough to be realistically achievable within a week or two of part-time contributions. Work items will now have a life cycle like this:

  • New items/ideas not yet fleshed out, not yet planned, and/or not yet prioritized.
  • Backlog for work items being documented.
  • Ready for documented work items that are ready to be worked on; items are assigned a sprint (not necessarily the next one), and convert into issues at this stage.
  • In Progress is work in progress; a branch is created for resolving that issue.
  • In Review is work items ready to be peer reviewed; a pull request is opened at that stage.
  • Done is when the work is merged into dev/next.
  • Delivered status is set when the work is merged into main.

Items/issues will be assigned a priority level:

  • Urgent is the highest priority level, for things that should be worked on before anything else.
  • High is for work that’s directly aligned with the objectives of the sprint it’s in.
  • Medium priority work could be delayed a sprint or two.
  • Low priority work doesn’t need to be in the current sprint, but would be nice to deliver anyway.

The priority level of any given issue likely evolves over time, particularly the lower-level ones.

In addition to status and priority, each draft issue / work item gets sized. Again this is meant to evolve over time: issues should become smaller over time as they are refined and documented and broken down into smaller tasks.

  • X-Large items represent a large development that should be broken down into smaller tasks.
  • Large items represent a significant development effort that can realistically be completed within a sprint by a single developer.
  • Medium items represent perhaps up to 2-3 days of effort.
  • Small items represent small tasks that can be completed in a few hours.
  • Tiny items represent tasks that should only take a few minutes: fixing a typo, adding a column to a database view or table, a configuration tweak, etc.

As of this writing, Sprint 1 is in its second half, and I’m still working on the RPC infrastructure:

“Project Cucumber” – it just had to be named that.

There’s a bit of history around the cucumber thing, and it involves two major contributors we lost and think about fondly all the time.

Comintern simply disappeared one day after a brief return, after a year under terms that essentially prevented him from working on open-source projects and hanging out in a public chat. We’re all hoping he’s all right and will eventually pop back.
We lost our beloved ThunderFrame to a fierce cancer in late 2018. Rest in peace, my friend. I got the Ctrl+Alt+Delete cushion covers you wanted me to have.

Discord Server

Rubberduck’s dev chat was always in a Stack Exchange room under the Code Review site. In fact it’s just a Code Review chat room we ended up [ab]using for this purpose. Back in 2014 I was very active on CR, and as a moderator on that site in 2015-2018 it made a lot of sense to keep it there.

But with 2-week sprints and a living backlog, we’re going to need more than SE chat to pull this off, and this is where Discord shines.

The general chat is fun, but a nice thing about Discord is that you can schedule events, like a public Sprint Review presentation every two weeks, followed by a Sprint Planning conversation among developers.

I’ll be hosting these events regularly, whether there’s an audience or not, whether other contributors are present or not.

Sprint Review

At each end of a sprint, we’re going to be going over what was done in the previous two weeks, and developers will present/demo their work. Since I’m doing sprint 1 by myself the first review will be me going over the solution structure and explaining the mechanisms and abstractions involved at a high level; reviews for sprint 2 and onward will likely involve more contributors, and things will get more and more exciting to present every time.

Sprint Planning

After the review concludes, developers convene to plan a realistically deliverable workload for the upcoming sprint. If we overshoot and under-deliver, we can always adjust the next sprint. If we over-deliver, we can always pull work items from upcoming sprints into the current one. So this conversation is about the work itself, whether there’s enough information in an issue for anyone at the table (or not!) to pick up and complete that task within two weeks, and whether the backlog is healthy or falling behind; if it’s falling behind, we take the time to talk about what needs to happen and outline work items to be drafted and refined during the sprint (I’ll be doing that backlog maintenance).

So yeah, Rubberduck3 is starting to feel very much like it’s just about to officially kick off, and Rubberduck as a project is entering a whole new phase, in continuous delivery mode.

Join us on Discord to keep up! (<~ feel free to share that link!)

Rubberduck 3.0: January Update

I intended to write about Rubberduck 3.0 progress last December, but things snowballed during the Holidays and here we are two-three weeks later and wow, time flies! Happy New Year dear readers (belatedly, I guess), 2023 is full of promises, and there are very nice things going on that I need to take a moment and share here.

Without any further ado, let’s clear the big news.

3 interlocked gears. Gear 1 is the largest, is labelled "add-in client", and drives gear 2 which is smaller and labelled "LSP server". Gear 2 drives gear 3, another smaller gear labelled "LocalDb server".

The main issues with Rubberduck have always been:

  • Memory consumption: Rubberduck consumes a lot of memory in the host process.
  • Instabilities related to COM interop: various tear-down issues with Office CommandBar and dockable toolwindows.
  • Poor VBIDE extensibility tooling and editor interactions.
  • Logs are difficult to use, it’s not clear what is happening in response to what – even when there’s only a single instance writing to the logs. Adding more logging means making things worse.

With v3 we’re addressing these long-standing issues by taking a number of design decisions early in the development process. These decisions were weighted against their downsides and alternatives, and probably make Rubberduck the first VBIDE add-in to implement a LSP Server for its purposes.

Language Server Protocol

For a while there have been discussions among Rubberduck devs about whether implementing LSP would be a feasible thing to do. It’s a protocol that formalizes all communications between a client (an IDE) and a language server that is used in modern IDEs such as Visual Studio and VSCode; twinBASIC implements it, and Rubberduck 3.0 will implement it too.

By moving all of the language-processing aspects out-of-process into a language server, we immediately tackle memory consumption issues: most of the CPU and memory resources Rubberduck 3.0 will use, are going to be outside of the add-in/host process.

With LSP in place, Rubberduck’s objective to bring editing VBA code in the Visual Basic Editor into the 21st century feels closer than ever.


Rubberduck’s LSP implementation will be split in two processes, as the LSP server process will be a client for another server process that will host a SQLite database. SQLite is a lightweight library many applications on many platforms (including mobile!) use to persist data between sessions. The database is a local .db file, and the database engine runs in-process. Rubberduck 3.0 will host a SQLite instance in its own server process, and the LSP server process will communicate with it through JSON-RPC, the same way the add-in communicates with the LSP server.

Instead of keeping hundreds of thousands of objects in memory for quick lookups, Rubberduck will write these objects to the database, and only fetch what it needs to work, which should tremendously help reduce the memory and processing footprint of the add-in host process. Using it as a log target (instead of text files) could reduce in-process disk I/O… and replace it with socket I/O and work happening out-of-process.

Cross-Process Communication

The add-in project has no reference to the server project in the Rubberduck solution, and the calls aren’t late-bound either. What’s happening here is different, and there are implications: Remote Procedure Call (RPC) communications occur through web sockets (WS), using a port between 1024 and 5000. As a result, we need to have Windows Defender Firewall open that port for us:

A screenshot of the moment I knew the socket server worked.

Since everything is local, the port only needs private networks permission to operate. We use JsonRPC to send data through that port, so we’re streaming the bytes of human-readable, plain text JSON.

This new client/server architecture enforces a much more decoupled and robust solution.


Telemetry is considered a potentially controversial feature: it will be completely disabled by default and will have to be selectively opted-in explicitly, but with everything becoming asynchronous, trace logging alone often does not suffice for troubleshooting. By implementing a proper telemetry model, we’re giving ourselves the tools to track a request and all actions that stem from it, across the multiple processes.

Since the project started, the only usage data we ever had was our own biased anecdotal usage: we haven’t the slightest idea of what features are under-used, what features are clearly everyone’s favorites, what inspections are most commonly fired, what inspections are disabled, whether inspections we release disabled by default are ever enabled, etc.

Whether enabled or not, Rubberduck 3.0 will collect detailed telemetry data, and store it locally in the SQLite database, by default clearing any existing data on startup: vital debugging information is present if it’s needed.

Ok I’m opting-in, what gives?

Opting into telemetry will allow a Rubberduck client to automatically upload the telemetry data to a future endpoint on (via https), where it will be persisted to a SQL Server database schema. Since there is no need for us to track any users, while still potentially extremely detailed, all telemetry data will be anonymous and impossible to track back to any particular user, computer, organization, or country. The transmitted telemetry data will only ever contain information that was explicitly allowed to be transmitted.

Time will tell how aggregated telemetry data can be used, but with enough data we (that includes you) could gain valuable insights on various points of interest:

  • Rubberduck feature usage statistics
  • LSP performance monitoring and troubleshooting
  • VBA language usage statistics, common issues

By transmitting some or all of your telemetry data, you’ll be helping make Rubberduck better for everyone, just by using it. However should you decide to not opt into it, we understand and respect your decision. Note that TraceTelemetry items are the trace logs, so transmitting them is exactly like sending us your log file for troubleshooting. I’ll make a separate post with all the details around pre-release time, and these features will be exhaustively documented on the website.


Having the LSP and Telemetry models is one thing, actually implementing them is another. Last time I said I was going to be focusing primarily on the Rubberduck Editor UI, and I did for a while: the editor was progressing very well and I was making very conclusive tests with an in-process parser when I took the decision to move the parser out-of-process.

I proceeded to read the entire LSP specification and implemented a model for it. Shortly after, I realized that we were potentially going to be running multiple instances of a LSP server at once, and it dawned on me that having as many instances of the SQLite database loaded in memory was not going to be globally efficient… so I decided to pull the SQLite database into its own dedicated server process.

The whole exercise demanded a lot of movement in solution projects and namespaces, but I’m very happy with the results: everything is in its place, and the actual add-in project is pretty much empty!

I started with the server implementation that’s the furthest from the add-in: the SQLite database server. This server speaks to LSP through JSON-RPC, but while Language Server Protocol formalizes how the add-in and the LSP talk to each other, I don’t have such a formal protocol for communications between the LSP and the database… so I’m basing most of it on what I learned with LSP.

How it’s going to work: you start Excel and hit Alt+F11 to bring up the VBE. The Rubberduck add-in gets loaded and starts up, then starts a LSP server process and initializes it. In turn the LSP server starts, and attempts to locate the database server. If the database process isn’t found, the LSP server starts one. The Excel/VBE/Rubberduck client process owns the LSP server process, but nobody owns the database server: when the database has disconnected its last client, it automatically shuts down.

The servers (both database and LSP) are console applications that run silently as background processes. In order to facilitate configuring them, and viewing/reviewing their respective inputs and outputs, I’ve written a small client console application that shows the server console content, lets you easily export it to text files or copy it to the clipboard, etc.

a screenshot showing the Rubberduck.DataServer client console application in the middle of exporting a log output to a .log text file.
Screenshot from before the DataServer UI was moved into its own LocalDbClient project.

The LSP client console application will have an additional Telemetry tab to review, delete, and manually submit telemetry data. Server log trace can be set to verbose or turned off, and the server itself can be instructed to shut down, directly from this application.

When RD3 releases, these client console applications will probably be accessible from an add-in menu, or perhaps they’ll be started together with the add-in and minimized to the system tray… we’ll cross that bridge when we get to the river.

Meanwhile work on the editor itself has taken a backseat, since it wasn’t useful to work on parameter info tooltips and wire up add-in functionality that would have to be later undone to work through the LSP server. All of the proof-of-concept stuff that worked, is still working. It just needs to be wired up to work with LSP requests and notifications, so focus has now shifted to the language server and its database backend.

The next few weeks/months are going to be all about implementing the LSP server, most likely.

Rubberduck.Fakes Gets an Upgrade

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:

NameDescriptionParameter names
MsgBoxConfigures VBA.Interaction.MsgBox callsFakes.Params.MsgBox
InputBoxConfigures VBA.Interaction.InputBox callsFakes.Params.InputBox
BeepConfigures VBA.Interaction.Beep calls
EnvironConfigures VBA.Interaction.Environ callsFakes.Params.Environ
TimerConfigures VBA.DateTime.Timer calls
DoEventsConfigures VBA.Interaction.DoEvents calls
ShellConfigures VBA.Interaction.Shell callsFakes.Params.Shell
SendKeysConfigures VBA.Interaction.SendKeys callsFakes.Params.SendKeys
KillConfigures VBA.FileSystem.Kill callsFakes.Params.Kill
MkDirConfigures VBA.FileSystem.MkDir callsFakes.Params.MkDir
RmDirConfigures VBA.FileSystem.RmDir callsFakes.Params.RmDir
ChDirConfigures VBA.FileSystem.ChDir callsFakes.Params.ChDir
ChDriveConfigures VBA.FileSystem.ChDrive callsFakes.Params.ChDrive
CurDirConfigures VBA.FileSystem.CurDir callsFakes.Params.CurDir
NowConfigures VBA.DateTime.Now calls
TimeConfigures VBA.DateTime.Time calls
DateConfigures VBA.DateTime.Date calls
Rnd*Configures VBA.Math.Rnd callsFakes.Params.Rnd
DeleteSetting*Configures VBA.Interaction.DeleteSetting callsFakes.Params.DeleteSetting
SaveSetting*Configures VBA.Interaction.SaveSetting callsFakes.Params.SaveSetting
Randomize*Configures VBA.Math.Randomize callsFakes.Params.Randomize
GetAllSettings*Configures VBA.Interaction.GetAllSettings calls
SetAttr*Configures VBA.FileSystem.SetAttr callsFakes.Params.SetAttr
GetAttr*Configures VBA.FileSystem.GetAttr callsFakes.Params.GetAttr
FileLen*Configures VBA.FileSystem.FileLen callsFakes.Params.FileLen
FileDateTime*Configures VBA.FileSystem.FileDateTime callsFakes.Params.FileDateTime
FreeFile*Configures VBA.FileSystem.FreeFile callsFakes.Params.FreeFile
IMEStatus*Configures VBA.Information.IMEStatus calls
Dir*Configures VBA.FileSystem.Dir callsFakes.Params.Dir
FileCopy*Configures VBA.FileSystem.FileCopy callsFakes.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
        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
        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:

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
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:

Option Explicit
Option Private Module
Private Assert As Rubberduck.AssertClass
Private Fakes As Rubberduck.FakesProvider
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
        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:

Public Sub TestMe_WhenPromptYes_GetsNextRandomValue()
    ' Arrange
    Fakes.MsgBox.Returns vbYes
    ' Act
    Module1.TestMe New LoggerStub ' ILogger is irrelevant for this test
    ' Assert
End Sub
Public Sub TestMe_WhenPromptNo_DoesNotGetNextRandomValue()
    ' Arrange
    Fakes.MsgBox.Returns vbNo
    ' Act
    Module1.TestMe New LoggerStub ' ILogger is irrelevant for this test
    ' Assert
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:

AssignsByRefConfigures the fake such as an invocation assigns the specified value to the specified ByRef argument.
PassthroughGets/sets whether invocations should pass through to the native call.
RaisesErrorConfigures the fake such as an invocation raises the specified run-time error.
ReturnsConfigures the fake such as the specified invocation returns the specified value.
ReturnsWhenConfigures the fake such as the specified invocation returns the specified value
given a specific parameter value.
VerifyGets 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:

AtLeastVerifies that the faked procedure was called a specified minimum number of times.
AtLeastOnceVerifies that the faked procedure was called one or more times.
AtMostVerifies that the faked procedure was called a specified maximum number of times.
AtMostOnceVerifies that the faked procedure was not called or was only called once.
BetweenVerifies that the number of times the faked procedure was called falls within the supplied range.
ExactlyVerifies that the faked procedure was called a specified number of times.
NeverVerifies that the faked procedure was called exactly 0 times.
OnceVerifies that the faked procedure was called exactly one time.
ParameterVerifies that the value of a given parameter to the faked procedure matches a specific value.
ParameterInRangeVerifies that the value of a given parameter to the faked procedure falls within a specified range.
ParameterIsPassedVerifies that an optional parameter was passed to the faked procedure. The value is not evaluated.
ParameterIsTypeVerifies 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.

Rubberduck 3.0 Progress Update

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!

Website News

As I wrote last July, I’ve started to get more time for myself lately, and that means I get to tackle a number of long-standing projects that have been on the backburner for way too long. One of them is the rewrite of the project’s website, which has been “under construction” ever since it was published as an ASP.NET MVC website, a few years ago already.

If you missed it, I tweeted a sneak-peek link last week:

Tweeted 09/28: “A couple of things need a bit of work still, but this website rewrite is coming along nicely – have a peek here:

Why a rewrite?

For the longest time, I wouldn’t have considered myself a web developer. I have well over a decade of experience in C# desktop development, but the web stuff essentially scared me to death. The version of the website that’s currently live was pretty much my first time doing anything like it. The site itself wouldn’t write to the database; it was another application that pulled the tag metadata, downloaded the xml-doc assets, parsed the documentation and examples, and wrote them to the database.

One of the biggest issues with the current model, is that the database is made to contain HTML that is needlessly difficult to modify:

Unreachable code is certainly unintended, and is probably either redundant, or a bug.
<p>The following quick-fixes are available for this inspection:</p>
<ul style="margin-left: 8px; list-style: none;">
<span class="icon icon-ignoreonce"></span>
<a href="">IgnoreOnce</a>
: Adds an '@Ignore annotation to ignore a specific inspection result. Applicable to all inspections whose results can be annotated in a module.</li> 
<span class="icon icon-tick"></span>
<a href="">IgnoreInModule</a>
: Adds an '@IgnoreModule annotation to ignore a inspection results for a specific inspection inside a whole module. Applicable to all inspections whose results can be annotated in a module.

Having this HTML markup, CSS classes, and inline styles as part of the data meant the data was being responsible for its own layout and appearance on the site. With the new JSON objects serialized into this Properties column, I could easily keep everything strongly typed and come up with separate view models for inspections, quick-fixes, and annotations, that each did their own thing and let the website in charge of the layout and appearance of everything.

Separation of Concerns

The solution architecture could be roughly depicted like this – I suppose I meant the arrows to represents “depends on” but note that this doesn’t necessarily mean a direct project reference: the Client/API relationship is through HTTPS, and no project in the solution references the Rubberduck.Database SQL Server database project, but ContentServices connects to a rubberduckdb database that you can deploy locally using that database project:

You could draw a thick red line between Rubberduck.Client and Rubberduck.API (actually that’s Rubberduck.WebApi now), and it would perhaps better illustrate the actual wall between the website and the data: the website project doesn’t need a connection string, only a base URL for the API!

Authentication is assured with GitHub’s API using OAuth2: if you authorize the rubberduck-vba OAuth application to your profile, the HttpContext.User is cast as a ClaimsPrincipal and claims the GitHub login as a name, and a rubberduck-org role claim is added when organization membership is validated; an additional rubberduck-admin role claim is added if the user is also a member of the WebAdmin org team.

The website packages the HttpContext.User into a Json Web Token (JWT), an encrypted string that encapsulates the claims; this token is passed as a bearer token in authenticated API requests. The API accepts an Authorize header with either such a bearer token, or a valid GitHub personal access token (PAT).

The API receives a request, and given an Authorization header, either decrypts the JWT or queries GitHub to validate the provided access token and attach the appropriate role claims, before any controller action is invoked.

Another authentication filter performs a similar task to authorize an incoming webhook payload: the rubberduck-webhook role is set and tag metadata and xml-doc content can get updated automatically whenever a new tag/release gets created.


This new website performs much, much better than the current one. It sends asynchronous (ajax) requests to the MVC controller to render partial views, fetching only enough information to paginate the data and present a decent preview. Since most pages are presenting markdown content, an asynchronous request is also sent to format the markdown and, if applicable, apply syntax highlighting to code blocks. At this stage static content isn’t being cached yet, and screenshots should be loaded dynamically – still, performance is quite decent:

Home page scores 94, but then both Code Inspections and Inspections pages (two pages with extensive content, lots of markdown, code blocks, etc.) score a full 100 with Google Lighthouse, so things are looking very good performance-wise.

Another detail: the code examples no longer trigger a page load when you select a tab, so everything just feels much smoother now. Note, as of this writing the example records have been wiped from the database while I work on fixing a problem with the xml-doc processing, so annotations, inspections, and quick-fixes aren’t showing any examples on the test site for now.

Online Indenter

This feature once worked, but then my inexperienced past self, went and broke it in an attempt to make it asynchronous. Well, it’s back online and running Rubberduck.SmartIndenter.dll version 2.5.2:

You can paste VBA code into the box there, click the Indent button, then copy the indented code back into the clipboard.

The code can be indented as per the default indenter settings (which are also used for indenting all syntax-highlighted code blocks on the site), or if you expand the Indenter Settings panel you can tweak every knob Rubberduck’s Smart Indenter port has to offer.

It wouldn’t be too hard to include a “download these settings” button here, to serialize the settings into a .xml file that Rubberduck can then import to update indenter settings.

Content Administration

Users with the appropriate claims will be able to see additional buttons and commands on the site:

A modal dialog allows authenticated users to add and edit markdown content directly on the site.

Content administration features still need a little bit of work, but they are already being used to document how to use each and every single feature in Rubberduck – once this documentation is completed, the site will be a huge user manual, and ready for launch!

What’s Next?

Once everything works as it should (getting very close now!) and all that’s left to do is to take screenshots and generate more content, I’ll shift my focus to the Rubberduck3 project, the ownership of which I’ve now transferred over to the rubberduck-vba organization – the repo remains private for now, but all Rubberduck contributors have access to it. Uploading the RubberduckWebsite solution as a public repository isn’t a priority at this point; I feel like dealing with the implications of having API secrets in a .config file would be a distraction that I don’t need right now. When the time comes, it’ll be properly setup with continuous integration and deployment, but there are other priorities for now.

Like this little guy…

Project planning has begun for Rubberduck 3.0

Untitled Icebreaker

So, I haven’t posted here for well over a year now, and Rubberduck ‘s last “green release” was in May of 2021 (18,773 downloads as of this writing – the download stats on the site’s front page are stale, as of last February). The website has been “under construction” forever, and all this time I barely answered or participated on Stack Overflow and have been pretty much a ghost in the project’s dev chat and GitHub repository.

Let’s call that a sabbatical. I mean, COVID lockdowns and 2020-2021 hit hard for pretty much everyone, so I’ll just say s..tuff happens and you just have to let things go and do your best to enjoy the ride regardless.

And I did! But as of 2022 July 1, I’m no longer a Microsoft MVP: I did not apply for a 4th renewal, because I know what MVPs do, and I know what I can do myself, and I can’t be honest with myself and pretend the little content I put up in 2021 makes a solid application worthy of consideration. It’s been an amazing experience, but I wouldn’t have been comfortable with accepting this annual award for what I did that year: the Microsoft MVP award means much more than that to me.

As of this writing, the repository has over 1.6K stars on GitHub, 275 forks, and a snapshot of it is even archived along with thousands of other public repositories in the GitHub Antarctic Vault! I’m proud of what we have accomplished, and proud to be able to say Rubberduck has made it beyond the wildest dreams we had for it back in 2015! Years fly… I haven’t written any VBA code in a while, but I wasn’t writing much VBA code anyway ever since C# has been putting the bread on my table, and that never stopped me (or anyone) from working on Rubberduck. I still have ideas and now & then think of where the project should be heading, and the time will definitely come when I can turn that into code again – just not now.

What about Rubberduck?

Rubberduck has come a long, a very long way, and we’ve learned a lot all along – devs and users alike! I kind of burned out though. Nothing seemed to ever make a dent in the growing mountain of issues, and the code base being quite daunting doesn’t make it particularly friendly to new contributors or would-be C# programmers coming from VBA: it was all going to be on pretty much just us to churn through it all. Like answering on Stack Overflow, it became a chore. I couldn’t write anymore, and figured the last post I wrote made a nice front page for the blog anyway.

I strongly doubt Rubberduck can get where we envisioned it for 3.0 in the current state of the project. We could get a v2.6 out there, perhaps up to 2.9 even, but we always wanted more out of a 3.0 build, and the scale and implications of the changes needed for that to happen are rather scary.

I want the next major version to do away with performance issues, from startup to teardown; I want a smaller memory footprint; asynchronous, continuous parsing; total control over what we get to show in a code pane… but we can’t have that without a complete overhaul of the entire architecture and a redesign of the deepest core mechanisms in Rubberduck.

So where’s Rubberduck headed as a project? On one hand we have a massive and mature code base with roughly 200 known issues (and over 700 unimplemented ideas); on the other hand we have a goal that cannot realistically be fulfilled incrementally from where we’re at. I think we need to start with a brand new shell, and bring in the original code piece by piece, adjusting them as we go.

Meanwhile twinBASIC is steadily progressing, but still isn’t quite ready to power a brand new language layer for Rubberduck yet, so we’re still stuck with our Antlr4 grammar and parser, and step one would be to implement a parser that can survive syntax errors so Rubberduck can still process code that cannot run. You see ever since we introduced Antlr4, we’ve been using its token positions to locate specific code in code panes; when a new line gets inserted or any part of any code moves around, a new parse is needed for that entire module to keep these tokens in sync with what’s in the VBE. This is what’s painted us into a corner of endless parse-edit-reparse cycles.

It’s not just the tokens, it’s also what we’ve built on top of it: the `Declaration` class has so many constructors with so many constructor parameters… everything that isn’t a comment, from a line label to variables and procedures, is a declaration. Over the years a decent inheritance hierarchy was built so we have many types of specialized declarations, but each and every one of them is a rather heavy object, and thousands upon thousands of them need to be spawned every time we parse so that the resolver can keep track of everything. Then the VBE is as always actively sabotaging our efforts by hiding attributes, so a complete parse must involve exporting every module to a temporary file that we parse for these hidden attributes, which then need to be associated with the correct declaration.

In order to happen at all, Rubberduck 3.0 needs to completely flip things around, so instead of synchronizing Rubberduck with the VBE, we’d be synchronizing the VBE with Rubberduck. The implications are pretty radical: the VBE gets reduced to a mere host, and Rubberduck needs to be the code editor now, instead of interacting (and fighting!) with the VBE. In other words, I hate to say this but Rubberduck can’t really go forward without essentially a full rewrite, and without implementing what amounts to a whole IDE, inside the VBE.

Known showstoppers include hooking up and interacting with the debugger: there is no API for this, so there wouldn’t be any way to track execution, hit breakpoints, and step through the source code in the custom editor; clearly it’s a sub-par user experience, but if it’s the best we can get at one point we’re going to have to bite the bullet and make it happen anyway, and have the legacy VBE take over when code is running or paused.

So what’s likely going to happen is that the 1.6K-stars repository will remain on the 2.x architecture, and a brand new repo will eventually be up for the 3.0 rewrite. This is obviously risky, because a “Grand Redesign in the Sky” rarely ends well, but I’m not seeing any other way. I guess we’ll just have to see where that goes. Worst-case, we scrap it, stick with the 2.x architecture, and let go of the dream.

What about the website?

Early this year I had a fully working .net 6 solution that I intended to host on Microsoft Azure, but the monthly fees would quickly have depleted the donations we get and it just would not have been sustainable at all, so I thought “hey no problem I’ll just deploy it to the current GoDaddy host”… but the latest version of .net that runs over there is Framework 4.8, which means the solution had to be re-targeted and many little pieces had to be reworked – that’s done now, but the Bootstrap stuff broke and the entire front-end makes no sense anymore and I haven’t really had a chance to spend much time fixing it since then.

Between getting the ball rolling on v3.0 and getting the new website online, my priority is on the website, because well, at the end of the day I’m paying for this hosting and https — many thanks to our donors for this!

The revamped website will have much less static content and pull feature pages from its database; members of the Rubberduck GitHub organisation can edit this content as needed, easily upload updated screenshots, etc. The idea is to make the website be a huge user manual that describes every single feature and sub-feature, without needing a new deployment every time a typo needs fixing. I can’t yet commit to an ETA, but as things in my life fall back into place I’ll start having more time for this in the next few weeks/months. Let’s just say I intend to have it up before the next edition of Hacktoberfest!

What about the blog?

I love writing, so I’m not done here. As a Microsoft MVP I ended up feeling pressured to produce regular content to justify my renewals, and that wasn’t healthy. As I usually took inspiration from various answering experiences on Stack Overflow, I didn’t want my first post in over a year to be just a casual one about programming in VBA: I needed an ice breaker of some sort, and this is it. I’ll be writing more regularly now, but at my own pace.

So yeah, I’m *slowly* coming back from a rather long hiatus with a lot of rather large projects on my plate, and I intend to not burn myself up doing any of it, so this will mean rigorously time-boxing my activities and keeping a sane balance between Rubberduck, the website, the blog, the MVVM library that I still intend to build with twinBASIC, but also with my day job and family life. I’ll be happy if that makes me a Microsoft MVP again in 2023, but it’s not what I’m shooting for.

Next post should be about the upcoming Rubberduck 2.5.3 green release, it’s well past due!

Rubberduck Style Guide

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.


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.

And I want Rubberduck to be like that, to be the companion tool that looks at your code and reveals bits of trivia, hints like “hey did you know this conditional assignment could be simplified?“, or “if that condition was inverted you wouldn’t need this empty block here“.

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 Count simply 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.


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.


  • 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.


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.


  • 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 a block 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.


  • 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 Type structure 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.


  • Use explicit modifiers everywhere (Public/Private, ByRef/ByVal).
  • Declare an explicit data type, even (especially!) if it’s Variant.
  • Avoid implicit qualifiers for all member calls: in Excel watch for implicit ActiveSheet references, implicit ActiveWorkbook references, implicit containing worksheet references, and implicit containing workbook references, as they are an extremely frequent source of bugs.
  • Invoke parameterless default members explicitly.
    • 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:

  1. Arrange dependencies and set expectations.
  2. Act, by invoking the method or function under test.
  3. 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:

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
'@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.


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…