VBA Interfaces and Polymorphism

Question

I am experimenting with VBA's Interfaces, and I'm a bit annoyed. Most of what I've read on the topic suggests using Interfaces as a means to achieve polymorphism, which seems to work in some of my use cases. Unfortunately, I have run into an issue.

I created a class module to define my interface iCanvasObject

I then created a class cTable that implements all methods in iCanvasObject. So far so good, everything works as expected.

The issue occurs when I define a method specific to cTable, and not part of iCanvasObject. Even if its Public I can't seem to access it from a variable dimensioned as iCanvasObject, but instantiated as cTable.

Dim tbl As iCanvasObject
Set tbl = New cTable

Its not visible in IntelliSense which I could live with, but when I try to simply call the method directly, VBA complains of a Method or data member not found error. Even though this variable is definitely of type cTable

Is there a way for me to create methods specific to cTable that I can call while still utilizing the polymorphism benefits of the interface? Said another way, if I dimension a variable as iCanvasObject, am I strictly limited to whats defined in this interface?

Answer 1

That's the way polymorphism works. If you declare it as iCanvasObject then that is what it is, and thus, it doesn't have any of the members of ctable that don't come from iCanvasObject, unless you cast it as a cTable.

This has nothing to do with the fact that you're using VBA.

Answer 2

The answer is simple, if you want access to methods that are not part of the interface you have 2 options:

Dim tbl As cTable

or Cast:

Dim tbl As iCanvasObject
Set tbl = New cTable

Dim tbl_i As CTable = DirectCast(tbl, cTable)

But I'm not sure VBA supports casting so option #1 is probably better.

Answer 3

If you need to access the cTable methods and properties, you will need to "cast" it back to a cTable temporarily. You can do this as follows:

Dim tbl As iCanvasObject
Set tbl = New cTable

'Access iCanvasObject methods/properties through tbl

Dim tempTable As cTable
Set tempTable = tbl 'Implicitly casts tbl to a cTable because tempTable is an object reference to a cTable.

'Access cTable methods/properties through tempTable

Answer 4

You will have to cast. The good news is that classes can implement multiple interfaces.

What I typically do in this situation is I include in my class definition an asSomeInterface method that returns an instance of the class as whatever interface the class is implementing.

Additionally, it should be noted that you can cast from one interface type to another, and VBA will allow this as long as the instantiated object implements both.