pokemon,

Assuming you have the interface defined as a ComImport in .NET, you can
implement it just like a .NET interface.

If you don't have the ComImport interface already (via an interop assembly
or some other .NET assembly), then either create an interop assembly from the
COM DLL or look at the interface definition in the IDL and manually create a
.NET interface that mimics it, something like this:

[ComImport]
[Guid("GUID_OF_INTERFACE")]
public interface IMyInterface
{
   // Returns HRESULT
   int Function1([In] int param1, [Out] out int param2);
}

Then implement it as a normal .NET interface.

Hope this helps.

Adam