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

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 is not thread-safe and requires help from an STA thread. CoCreateInstance() uses this value when it creates the object. If necessary it will start an STA thread and create a proxy if the current thread is not STA, ensuring it is always used in a thread-safe way.

A [ComVisible] .NET class will be registered as "Both", indicating that it is okay to be used on a thread in the MTA. Pretty optimistic, but follows the .NET philosophy that everything is thread-unsafe but can be made safe by putting the lock keyword in the right places. A promise that is not often tested btw, risky. Overriding the ThreadingModel value (or omitting it) requires writing code to register the coclass yourself, decorated with the [ComRegisterFunction] attribute. RegistrationServices.RegisterTypeForComClients() can be useful to get the basic keys into place.