Hi

I think that would be a large project.
Because .NET did not have most COM interface declare, but the Active
Document host will need to be concern with many COM interactive. We need to
declare all the interface carefully ourselves. Also it may be concern with
many unmanaged OLE interface and memory operation, we need many unmanaged
call.

The Application class is used in winform application to handle the message
loop. We did not expose it.
Commonly if we wants to expose a .NET class, we just need to create a Class
Library project and check the COm Interop in the project setting.

If you want to have a start with COM interop, you may start from the
article here.
Exposing .NET Framework Components to COM
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/htm
l/cpconexposingnetframeworkcomponentstocom.asp

Best regards,

Peter Huang
Microsoft Online Partner Support