Is it possible to hide a class parent when exported to COM with ComVisible?

Question

In a C# class library (.Net 3.5), I have a very simple class (some string and long properties, no methods) which inherit from System.Web.Services.Protocols.SoapHeader. My class is tagged as ComVisible(true) because I need to use it in Delphi.

For some unknown reason, this class can't be created in Delphi, when the class interface is automatically exported (aka attribute ClassInterface(ClassInterfaceType.AutoDispatch)). I made a simple C++ program to test about that, and I get each time the same error: 0x80131509 (Methods parameters or return type cannot be invoked via IDispatch). After some digging, the problem is directly related to SoapHeader. When I remove it or use it as property of my class, all run fine.

In C++, when using IDispatch to access my class exported with ClassInterfaceType.None, all work fine. But I can't relay on IDispatch in Delphi, so I'm searching a way to avoid the type library exporter to export SoapHeader stuff.

Is there a way to do that ? A .Net attribute to inform the type library exporter to avoid exposing a class parent ? To expose only a part of a class even if there is more stuff ?

What exactly did you intend to expose through the interface? Native COM interfaces (non-IDispatch) probably expect a non-empty vtable, and you have zero methods on it. So did you try adding methods named getProperty1' and getProperty2` instead of relying on properties to magically work without any vtable entries for the get property methods? — Warren P

Hans Passant Hans Passant · Accepted Answer · 2011-08-05T15:34:52

Declare an interface so you can explicitly name the members that are visible to COM clients. Like this:

using System;
using System.Runtime.InteropServices;

[ComVisible(true)]
[InterfaceType(ComInterfaceType.InterfaceIsDual)]
public interface IThingsIWantToExpose {
    void mumble();
    // etc..
}

[ComVisible(true)]
[ClassInterface(ClassInterfaceType.None)]
public class MyComClass : SomeBaseClass, IThingsIWantToExpose {
    // Actual implementation of the methods
    public void mumble() { }
    // etc, plus everything else that you want to do in the class
    //...
}

Using ComInterfaceType.InterfaceIsDual allows both early and late binding (IDispatch). ClassInterfaceType.None hides the class members so the base class isn't exposed either. Including the System.Object members btw.

Is it possible to hide a class parent when exported to COM with ComVisible?

1 Answers