How to mark .NET objects exposed to COM-interop as single threaded?

Question

When defining a COM-visible class in C++ I can define the supported threading model in the header file (the threading(single) line):

[
    coclass,
    default(IComInterface),
    threading(single),
    vi_progid("Example.ComClass"),
    progid("Example.ComClass.1"),
    version(1.0),
    uuid("72861DF5-4C77-43ec-A4DC-ED04396F0CCD")
]

Is there a comparable way of setting the threading model in .NET (for example an attribute)? I currently define my COM-class as such:

[Guid("67155A91-2948-43f5-B07F-5C55CDD240E5")]
[ComVisible(true)]
[InterfaceType(ComInterfaceType.InterfaceIsDual)]
public interface IComInterface
{
    ...
}


[Guid("DC5E6955-BB29-44c8-9FC0-6AADEEB2AFFB")]
[ClassInterface(ClassInterfaceType.None)]
[ProgId("Example.ComClass")]
public class ComClass : IComInterface
{
    ...
}

--edit:

The comments on the marked answer are the really important thing. It seems like the only way to tell RegAsm to set a different ThreadingModel is to write a custom registration method marked with the [ComRegisterFunction]attribute.

Answer 1

That's really obscure, I've never seen the "threading" attribute in MIDL. Nor have the MSDN Library authors.

A COM coclass publishes its threading requirements in the registry, using the HKCR\CLSID{guid}\InProcServer32 key. The ThreadingModel value declares the apartment it needs. If it is missing or is set to "Apartment" then is announces that it needs STA. CoCreateObject() uses this value when it creates the object. It will create a proxy if the current thread is not STA.

Writing custom registry values is a bit awkward in .NET, you'd have to use the [ComRegisterFunction] attribute and take over the duties of Regasm.exe.