vb.net - Generics and Com Visible .NET libraries

Question

I've developed a VB.NET library (partially developed on C# as well) that heavily depends on inheriting from an abstract generic base class, and I'm trying to figure out the best practice for this. I Sadly have to do it using framework 3.5.

Public MustInherit Class MyBaseClass(Of T)
  Public Whatever As T
End Class

Public Class MyDerivedClass
  Inherits MyBaseClass(Of String)

  Private _myProperty As String

  Public Property MyProperty As String
    Get
      Return _myProperty
    End Get
    Set(value As String)
      _myProperty = value
    End Set
  End Property
End Class

I attach the .tlb file as a reference in VBA (using Excel), and I run the following code:

Dim m As New VBtoVBA.MyDerivedClass
m.MyProperty = "foo"

And I get the error "Run-time error 430: Class does not support Automation or does not support expected interface".

On the other hand, I change the first lines to:

Public MustInherit Class MyBaseClass
  Public Whatever As String
End Class

Public Class MyDerivedClass
  Inherits MyBaseClass

The VBA script works. Hence I assume the issue is with generics (as documented in other sources as well). Dropping the generic feature of my library is not possible, though. The "best" workaround I can think of is to write a third class that includes MyDerivedClass as a field, and works as a non-generic interface to it:

Public Class MyDerivedClassString

  Private _innerObj As New MyDerivedClass

  Public Property MyProperty As String
    Get
      Return _innerObj.MyProperty
    End Get
    Set(value As String)
      _innerObj.MyProperty = value
    End Set
  End Property

  Public Property Whatever As String
    Get
      Return _innerObj.Whatever
    End Get
    Set(value As String)
      _innerObj.Whatever = value
    End Set
  End Property

End Class

This way I can work with it pretty much as I'd like to in VBA:

m.Whatever = "wha"
MsgBox (m.Whatever)

By the way I think that there might be another (better) way to achieve the same result, and i really hope so since m library is made up of dozens of classes.

Many thanks.

Answer 1

As i mentioned in comment writing library (dll) for MS Office applications is bit... hard-coded. This dll must exposes methods and properties to be able to use it in COM automation. To be able to achieve that, you need to write interface(s):

Namespace VBtoVBA
    Public Interface IMyDerivedClass
        Property MyProperty As String
    End Interface
End Namespace

then in DerivedClass

Public Class MyDerivedClass
    Inherits MyBaseClass(Of String)
    Implements IMyDerivedClass

    Private _myProperty As String

    Public Property MyProperty As String Implements IMyDerivedClass.MyProperty

Now, go to Project Properties window 1) choose Application tab - click on Assembly Information button and in the next window select Make assembly COM visible checkbox (apply setting using OK button),

2) choose Compile tab - select Register for COM interop checkbox

3) Save project and Build dll

4) Now, go to VBA Code editor in Office application -> References menu. In Reference window add reference to yourDllName.tlb

Now, you can use your dll in Office application ;)

I tested code:

Option Explicit

Sub DoSomething()
Dim m As VBtoVBA.MyDerivedClass

Set m = New VBtoVBA.MyDerivedClass

m.MyProperty = "Something"

MsgBox m.MyProperty


End Sub

and it works as well ;)