help: Call C# winforms dll from VB6 project?

Question

I have a VB6 project(windows application) and I have to redevelop a module in the existing VB6 project in C#.net.

The module that I develop in C#.net should be a dll an

Answer 1

The only way to do it is to expose your C# class as a COM object (also called a CCW - COM Callable Wrapper), and create an instance of that COM object in your VB6 code.

This should help you get started:

http://www.bing.com/search?q=C%23+CCW&go=&form=QBRE&qs=n

Answer 2

There some excellent help on msdn here:

http://msdn.microsoft.com/en-us/library/6bw51z5z.aspx

Answer 3

You have to make your class COM-Visible. Here's how I would change your code:

namespace TestDll
{
    [Guid("FB8AB9B9-6986-4130-BD74-4439776D1A3D")]
    [InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
    [ComVisible(true)]
    public interface IClass1
    {
        [DispId(50)]
        void DisplayMessage();
    }


   [Guid("74201338-6927-421d-A095-3BE4FD1EF0B4")]
   [ClassInterface(ClassInterfaceType.None)]
   [ComVisible(true)]
   [ProgId("TestDll.Class1")]
    public class Class1:IClass1
    {              
        void IClass1.DisplayMessage()
        { 
            MessageBox.Show ("Displyaing message");
        }

    }
}

Note the [DispId(50)]. You want to specify the dispatch ID for your COM-visible methods, properties, and events. If you don't, the compiler will do it for you and you may end up breaking compatibility every time you compile. The number doesn't matter so much as it doesn't change between compiles.

You might want to check out Building COM Objects in C#. It's a pretty good getting started tutorial.

Some highlights:

Exposing the VC# objects to the COM world requires the following …

* The class must be public
* Properties, methods, and events must be public.
* Properties and methods must be declared on the class interface.
* Events must be declared in the event interface.

Every Interface needs a GUID property set before the interface name. To generate the unique Guid , use the guidgen.exe utility and select the Registry Format.