Inheritance and COM

10 Jun 2010 8:03 PM

Alan Sloan

I have a Class Library that I am writing that is COM visible to maintain a
VB6 app. In this Class Library I have the following classes.

Public Class SomeBaseObject

    Public Function DoSomething() as Boolean
        .....
    End Function
    ....
End Class

<ComClass(Item.ClassId, Item.InterfaceId, Item.EventsId)> _
Public Class MyDerivedClass
    Inherits SomeBaseObject

    ' These GUIDs provide the COM identity for this class
    Public Const ClassId As String = "95af8113-4a12-42e4-b6f9-8beb9bdf7591"
    Public Const InterfaceId As String =
"1f7e7d99-d1c0-4732-8d2d-20c258d4883f"
    Public Const EventsId As String = "de109a8b-ad82-4ed7-8442-087d2ba569c3"

    ....
End Class

When I use MyDerivedClass in a .net assy, I can instantiate the
MyDerivedClass object and the method DoSomething is visible and available as
you would expect.

When I use MyDerivedClass in my VB6 app, I can instantiate the
MyDerivedClass "object" but I cannot see the method DoSomething(or anything
else) from the base class. All I can see or access are the properties and
methods of the MyDerivedClass object.

What am I missing? Any help would be greatly appreciated!
Alan

10 Jun 2010 11:29 PM

Armin Zingler

Am 10.06.2010 22:03, schrieb Alan Sloan:
Show quoteHide quote

> I have a Class Library that I am writing that is COM visible to maintain a
> VB6 app. In this Class Library I have the following classes.
>
> Public Class SomeBaseObject
>
>     Public Function DoSomething() as Boolean
>         .....
>     End Function
>     ....
> End Class
>
> <ComClass(Item.ClassId, Item.InterfaceId, Item.EventsId)> _
> Public Class MyDerivedClass
>     Inherits SomeBaseObject
>
>     ' These GUIDs provide the COM identity for this class
>     Public Const ClassId As String = "95af8113-4a12-42e4-b6f9-8beb9bdf7591"
>     Public Const InterfaceId As String =
> "1f7e7d99-d1c0-4732-8d2d-20c258d4883f"
>     Public Const EventsId As String = "de109a8b-ad82-4ed7-8442-087d2ba569c3"
>
>     ....
> End Class
>
> When I use MyDerivedClass in a .net assy, I can instantiate the
> MyDerivedClass object and the method DoSomething is visible and available as
> you would expect.
>
> When I use MyDerivedClass in my VB6 app, I can instantiate the
> MyDerivedClass "object" but I cannot see the method DoSomething(or anything
> else) from the base class. All I can see or access are the properties and
> methods of the MyDerivedClass object.
>
> What am I missing? Any help would be greatly appreciated!
> Alan

The ComClass attribute is the "quick" way to have interfaces created, but as you
see, it has it's drawbacks.

Inm VB6, you can still assign a MyDerivedClass object to a variable of type
SomeBaseObject because the object implements the base class interface:

    dim var as SomeBaseObject

        set var = new MyDerivedClass
        var.DoSomething        'Works

I prefer to have no interface created automatically. Instead, i implement them
explicitly. Have a look at the ClassInterface attribute. Instead of creating
example code by myself, this seems to be an explaining discussion about it:
http://social.msdn.microsoft.com/forums/en/csharpgeneral/thread/7313191a-10db-4a16-9cdd-de9fb80b378a/

--
Armin