Question

EDIT: I’ve written the results up as a blog post.

---

The C# compiler treats COM types somewhat magically. For instance, this statement looks normal…

Word.Application app = new Word.Application();

… until you realise that Application is an interface. Calling a constructor on an interface? Yoiks! This actually gets translated into a call to Type.GetTypeFromCLSID() and another to Activator.CreateInstance.

Additionally, in C# 4, you can use non-ref arguments for ref parameters, and the compiler just adds a local variable to pass by reference, discarding the results:

// FileName parameter is *really* a ref parameter
app.ActiveDocument.SaveAs(FileName: "test.doc");

(Yeah, there are a bunch of arguments missing. Aren’t optional parameters nice? 🙂

I’m trying to investigate the compiler behaviour, and I’m failing to fake the first part. I can do the second part with no problem:

using System;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

[ComImport, GuidAttribute("00012345-0000-0000-0000-000000000011")]
public interface Dummy
{
    void Foo(ref int x);
}

class Test
{
    static void Main()
    {
        Dummy dummy = null;
        dummy.Foo(10);
    }
}

I’d like to be able to write:

Dummy dummy = new Dummy();

though. Obviously it’ll go bang at execution time, but that’s okay. I’m just experimenting.

The other attributes added by the compiler for linked COM PIAs (CompilerGenerated and TypeIdentifier) don’t seem to do the trick… what’s the magic sauce?

Answer 1

By no means am I an expert in this, but I stumbled recently on what I think you want: the CoClass attribute class.

[System.Runtime.InteropServices.CoClass(typeof(Test))]
public interface Dummy { }

A coclass supplies concrete
implementation(s) of one or more
interfaces. In COM, such concrete
implementations can be written in any
programming language that supports COM
component development, e.g. Delphi,
C++, Visual Basic, etc.

See my answer to a similar question about the Microsoft Speech API, where you’re able to “instantiate” the interface SpVoice (but really, you’re instantiating SPVoiceClass).

[CoClass(typeof(SpVoiceClass))]
public interface SpVoice : ISpeechVoice, _ISpeechVoiceEvents_Event { }