Myujth

Context

I have a parent interface, IParent,

Option Explicit

Public Sub DoParentStuff()
End Sub

a child interface implementing IParent, IChild,

Option Explicit

Implements IParent

Private Sub IParent_DoParentStuff()
End Sub

Public Sub DoParentStuff()
End Sub

and a concrete implementation of IChild, CStandardChild.

Option Explicit

Implements IChild

Private Sub IChild_DoParentStuff()
End Sub

Public Sub DoParentStuff()
 IChild_DoParentStuff
End Sub

I then created a module that passes a variable of type IChild to a subroutine with one parameter of type IParent.

Option Explicit

Private Sub Test(ByRef parent As IParent)
 parent.DoParentStuff
End Sub

Public Sub Main()
 Dim child As IChild

 Set child = New CStandardChild

 Test child
End Sub

I can compile the VBA project without error. But, when I run Main, I get a run-time error

Run-time error '13':

Type mismatch

The debugger points to the code Test child.

Question

Why am I getting a run-time, type-mismatch error? How can I pass child to Test() without getting this error?

What I've Tried

I've looked into casting IChild to IParent. However, I'm not using VB.NET, so, I don't have access to DirectCast and CType. In saying this, if I've implemented IParent and IChild appropriately, I didn't think a cast is necessary.

If I understand correctly what you're trying to do, it looks like you're trying to extend IParent with IChild members. You can't do that in VBA - it would be awesome, but it's part of what makes .NET a more flexible framework to work with.

To take a C# analogy - this is what I think you're trying to do (and is illegal in VBA):

interface IFoo void DoSomething() 
interface IBar : IFoo void DoStuff() // inherits members of IFoo

If you need CStandardChild to be accessible through both IParent and IChild interfaces, you need both Implements statements in that class:

Option Explicit
Implements IChild
Implements IParent

'implement members of both interfaces...

Then you can pass an instance of that class and "cast" it to either interface.