The no-frills guide to .Net COM interop

So there a couple of guides out there that already cover some aspects of this post, but none of them cover all of the topics and many of them are convoluted. When I publish something to COM I usually end up consuming it via a scripting language like Python, Perl or VBScript so this little howto will reflect what I have found to be best practices. So here is the no-frills guide to exposing a C# class to COM.

• Create an interface
• Create a class that inherits from the interface
• Add GUIDs to both interface and class using the "Guid" attribute define in "System.Runtime.InteropServices". You can use the GUID generation tool found in the "Tools" menu of Visual Studio.

using System;
using System.Runtime.InteropServices;

namespace MyNamespace{

    [Guid("EFB47A26-D84F-4092-927D-232E92FB77E8")]
    public interface IMyComponent{
        [DispId(1)]
        void DoStuff();
    }

    [Guid("8F3DFCDB-45F7-461d-9B19-B140CD6F05FA")]
    public class MyComponent : IMyComponent{
        public void DoStuff() { }
    }
}

• Add the interface type to the interface, you don't need to know all the details but you should probably use the Dispatch type if this component is being consumed by a dynamically typed language such as Python.

• Add the DispId to each method exposed by the interface. Again this helps when being consumed by Python. A note of caution though, that once these values are set and you register the component they should not change. It's not the end of the world if they do, but you will cause headaches for yourself.

[Guid("EFB47A26-D84F-4092-927D-232E92FB77E8")]
[InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
public interface IMyComponent {
    [DispId(1)]
    void DoStuff();
}

• Add the ProgId attribute to the class, set the value to Namespace.ClassName. This is again one of those things that may not be necessary but helps when using Python, Perl etc...
• Add the ClassInterface attribute to the class setting the value to "ClassInterfaceType.AutoDispatch"
• Add the ComDefaultInterfaceAttribute setting the value to the type of your interface. Again, not neccessarily required but Python has all sorts of trouble generating the wrapper classes if it isn't present.

[ProgId("MyNamespace.MyComponent")]
[ClassInterface(ClassInterfaceType.AutoDispatch)]
[ComDefaultInterfaceAttribute(typeof(IMyComponent))]
[Guid("8F3DFCDB-45F7-461d-9B19-B140CD6F05FA")]
public class MyComponent : IMyComponent {
  public void DoStuff() { }
}

You need to enable two options in Visual Studio for your project. Open the properties for your project, and go the the application tab. Click on the "Assembly Information" button, and enable the make COM visible option. On the build tab you will also need to enable the register for com interop option at the bottom. You will also need to sign your assembly; you can use a self generated key through the signing tab of your project's properties. Once compiled Visual Studio will auto-register the assembly with COM so that you can test the interface with a script or OLEView. You can use regasm or build a Wix project to have it installed permanently.

using System;
using System.Runtime.InteropServices;

namespace MyNamespace
{

    [Guid("EFB47A26-D84F-4092-927D-232E92FB77E8")]
    [InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
    public interface IMyComponent{
        [DispId(1)]
        void DoStuff();
    }

    [ProgId("MyNamespace.MyComponent")]
    [ClassInterface(ClassInterfaceType.AutoDispatch)]
    [ComDefaultInterfaceAttribute(typeof(IMyComponent))]
    [Guid("8F3DFCDB-45F7-461d-9B19-B140CD6F05FA")]
    public class MyComponent : IMyComponent{
        public void DoStuff() { }
    }
}

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