Hi Jerry,

Normally a wrapper use composition instead of inheritance to hold a
reference to another class, and create some public methods to delegate the
calls into the wrapper class. In the example mentioned in article
(http://msdn2.microsoft.com/en-us/library/ms364069(VS.80).aspx#vbbestprac_to
pic4), the GuidWrapper is not inehriting from .NET Guid, instead, it
creates a public method NetGuid() and delegates the call to Guid.NewGuid().

That is not to say we cannot use inheritance to delegate the calls, you
certainly could create a new method and use "MyBase.Test()" to call into
the base class.

To better understand why inheriting from a .NET class directly doesn't
expose its public interface to COM client, we need to understand what is
the ComClassAttribute attribute doing for you.

The ComClassAttribute attribute instructs the compiler to add metadata that
allows a class to be exposed as a COM object.

Use ComClassAttribute to simplify the process of exposing COM components
from Visual Basic. COM objects are very different from .NET Framework
assemblies; without the ComClassAttribute, you need to follow a number of
steps to generate a COM object from Visual Basic. For classes marked with
ComClassAttribute, the compiler performs many of these steps automatically.

Here's a COM object without using the ComClassAttribute:

Imports System.Runtime.InteropServices

<Guid(ComClass3.InterfaceId)> _
Public Interface IComClass3
   Sub Test()
End Interface

<Guid(ComClass3.ClassId)> _
<ClassInterface(ClassInterfaceType.None)> _
Public Class ComClass3
   Implements IComClass3

   Public Const ClassId As String = "decd3406-2c6b-4335-86fe-0fb3ce3ac17c"
   Public Const InterfaceId As String =
"4222b51e-b9b7-4305-9c13-3bf2c3f1a56c"

   Public Sub Test() Implements IComClass3.Test

   End Sub
End Class

This is strictly following COM programming rules: we first define an
interface, then we define a coclass which implements the interface.

Now back to the issue you're seeing: Logging isn't applied
ComClassAttribute, there's no COM interface defined for it, the VB6 client
will not be able to see the public methods defined in it.

I hope this could help you better understand the COM interop. For your
reference, I'm including some good resources on this topic:

#Understanding Classic COM Interoperability With .NET Applications - The
Code Project - .NET
http://www.codeproject.com/dotnet/cominterop.asp

#Exposing .NET Components to COM - The Code Project - .NET
http://www.codeproject.com/dotnet/nettocom.asp

#Exposing COM interfaces of a .NET class library for Late Binding - The
Code Project - VB.NET
http://www.codeproject.com/vb/net/MusaExposingCOM.asp

#Chapter 3: Recommendations for Managed and Unmanaged Code Interoperability
http://msdn2.microsoft.com/en-us/library/ms993883.aspx

Hope this helps.

Regards,
Walter Wang (wawang@online.microsoft.com, remove 'online.')
Microsoft Online Community Support

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