<snip>

Thanks for looking into this, but I'm afraid I missed out some key
information. The abstract base class needs to implement a COM interface, and
it is this that is causing the problem. Here is a more complete example:

***Class1.cs***

using System;
using System.Runtime.InteropServices;

namespace ClassLibrary1
{

 [
   ComImport(),
   Guid("0000010C-0000-0000-C000-000000000046"),
   InterfaceType(ComInterfaceType.InterfaceIsIUnknown)
 ]
 public interface UCOMIPersist
 {
   void GetClassID(out Guid clsid);
 }

 [ComVisible(false)]
 public abstract class Class1 : UCOMIPersist
 {

   protected Class1() : base()
   {
   }

   public abstract Guid ClassId
   {
     get;
   }

   void UCOMIPersist.GetClassID(out Guid clsid)
   {
     clsid = ClassId;
   }

 }

}

***Class2.vb***

Option Explicit On
Option Strict On

Imports System
Imports System.Runtime.InteropServices

Namespace ClassLibrary2

 < _
   ClassInterface(ClassInterfaceType.None), _
   ComVisible(True), _
   Guid("4651D5FA-573D-42dd-8C34-07202759A5C2"), _
   ProgId("ClassLibrary2.Class1.1.0") _
 > _
 Public NotInheritable Class Class1
   Inherits ClassLibrary1.Class1

   Public Sub New()
     MyBase.New()
   End Sub

   <ComVisible(False)> _
   Public Overrides ReadOnly Property ClassId() As Guid
     Get
       Return New Guid("4651D5FA-573D-42dd-8C34-07202759A5C2")
     End Get
   End Property

 End Class

End NameSpace

Implementing the UCOMIPersist interface in the C# class means that the VB
class cannot be registered without a type library for the C# assembly.
Unfortunately, enabling "Register for COM Interop" on the C# assembly
results in the following build error: "COM Interop registration failed.
There are no registrable types in the built assembly." Running regasm
manually on the VB assembly with /tlb results in a type library being
created for the C# assembly automatically, and therefore regasm succeeds. Is
there any way I can avoid the requirement of a type library for the C#
assembly or at least be able to create it as part of the build using
"Register for COM Interop"?