Inherited interface not seen by VBA client

[Neil Sargent]

I am writing a COM Add-in for Microsoft Access 2010 using Visual Basic 2010.

I do not understand why the members of an inherited interface are not available to VBA. They do not appear in the VBA Object Browser and a run-time error occurs if an attempt is made to use them.

Here is a full code sample which demonstrates the problem:

First I have a base class with a defined interface:

Public Interface IClassBase
    Property id As String
    Property Name As String
End Interface
 
<ClassInterface(ClassInterfaceType.None)> _
Public Class ClassBase
    Implements IClassBase
 
    Private mstr_id As String
    Private mstr_Name As String
 
    Public Property id As String Implements IClassBase.id
        Get
            id = mstr_id
        End Get
        Set(ByVal value As String)
            mstr_id = value
        End Set
    End Property
 
    Public Property Name As String Implements IClassBase.Name
        Get
            Name = mstr_Name
        End Get
        Set(ByVal value As String)
            mstr_Name = value
        End Set
    End Property
End Class
 

Next I inherit the base class and interface into a derived class:

Public Interface IClassDerived
    Inherits IClassBase
    Property extra As String
End Interface
 
<ClassInterface(ClassInterfaceType.None)> _
Public Class ClassDerived
    Inherits ClassBase
    Implements IClassDerived
 
    Private mstr_extra As String
 
    Public Property extra As String Implements IClassDerived.extra
        Get
            extra = mstr_extra
        End Get
        Set(ByVal value As String)
            mstr_extra = value
        End Set
    End Property
End Class
 

This works fine within Visual Basic .NET. However in VBA, only the "extra" member of the derived class is visible. The "id" and "Name" properties are missing from the COM library interface.

The following code works around the problem. I make the members of the base class overridable:

Public Interface IVBAClassBase
    Property id As String
    Property Name As String
End Interface
 
<ClassInterface(ClassInterfaceType.None)> _
Public Class VBAClassBase
    Implements IVBAClassBase
 
    Private mstr_id As String
    Private mstr_Name As String
 
    Public Overridable Property id As String Implements IVBAClassBase.id
        Get
            id = mstr_id
        End Get
        Set(ByVal value As String)
            mstr_id = value
        End Set
    End Property
 
    Public Overridable Property Name As String Implements IVBAClassBase.Name
        Get
            Name = mstr_Name
        End Get
        Set(ByVal value As String)
            mstr_Name = value
        End Set
    End Property
End Class
 

I declare each member of the base class in the interface for the derived class and override each member of the base class:

Public Interface IVBAClassDerived
    Property id As String
    Property Name As String
    Property extra As String
End Interface
 
<ClassInterface(ClassInterfaceType.None)> _
Public Class VBAClassDerived
    Inherits VBAClassBase
    Implements IVBAClassDerived
 
    Private mstr_extra As String
 
    Public Property extra As String Implements IVBAClassDerived.extra
        Get
            extra = mstr_extra
        End Get
        Set(ByVal value As String)
            mstr_extra = value
        End Set
    End Property
 
    Public Overrides Property id As String Implements IVBAClassDerived.id
        Get
            id = MyBase.id
        End Get
        Set(ByVal value As String)
            MyBase.id = value
        End Set
    End Property
 
    Public Overrides Property Name As String Implements IVBAClassDerived.Name
        Get
            Name = MyBase.Name
        End Get
        Set(ByVal value As String)
            MyBase.Name = id
        End Set
    End Property
End Class
 

The problem with this method is that I have to override every member of the base class just to get them into the interface for the derived class. Since the members are mainly passive properties, I have lost most of the advantages of inheriting the base class.

Am I missing something? Why does VBA not recurse into the inherited interfaces correctly? Is there a way of telling the type library exporter to expand the inherited interfaces?