Expose a class as STA COM

Question

I have a little c# class that I need to expose as COM. THe tool that will use the COM object requires that I support STA. Is there anything special that I need to do with my COM object?

(I know about the STAThread attribute, but (unless I'm wrong) It is for indicating that a thread of execution in .net is to be STA compatible, and not for indicating that my COM object needs to be compatible).

Here's my declaration so far. Do you see anything that I need to add?

    [ComVisible(true)]
    [ClassInterface(ClassInterfaceType.None)]
    [ComDefaultInterface(typeof(ITestClass1))]
    [Guid("093A3701-2C53-4A31-97C5-0F3C205E5B4C")]
    public class Class1: ITestClass1 {..}

    [ComVisible(true)]
    [Guid("8D52F785-CDD8-4248-8AB7-97B8C19DE59B")]
    public interface ITestClass1 {..}

Answer 1

A COM server advertises the threading model it requires with a registry entry named ThreadingModel. STA is the default if the registry key is missing or when it is set to "Apartment".

Regasm.exe sets this key to "Both". Which means that your server is declared to be compatible with both STA threads and MTA threads. Changing this is very awkward, you have to write your own registration procedure and annotate it with the [ComRegisterFunction] attribute.

There's little point in actually doing so. If you declare your server to be apartment threaded then there will be problem with the client creates an instance on an MTA thread. The client needs a proxy/stub DLL, required to marshal calls from the MTA thread to the app's STA thread. You cannot create this DLL with .NET, it requires writing an .idl file and building an unmanaged DLL. Quite unpractical.

The better approach is leave the key set to "Both" and to check the apartment state in your class constructor. Use Thread.GetCurrentThread().GetApartmentState(). If you get MTA then throw an exception to tell the client that you don't support multi-threading.

Long story short: you don't have to do anything special if your tool requires STA support. All COM servers support that.