Why shouldn't gui threads live in multithreaded apartments?

Question

The COINIT - an enumeration used to specify whether a windows thread is in a single or multithreaded apartment - documentation (http://msdn.microsoft.com/en-gb/library/windows/desktop/ms678505(v=vs.85).aspx) states that:

The multi-threaded apartment is intended for use by non-GUI threads. Threads in multi-threaded apartments should not perform UI actions. This is because UI threads require a message pump, and COM does not pump messages for threads in a multi-threaded apartment.

Why shouldn't threads in multi threaded apartments perform UI actions? What is wrong with having a message loop in a thread in a multi threaded apartment? Does COM somehow provide an automatic message loop for a thread in a single threaded apartment?

Answer 1

That's a bit backwards, a UI thread primarily requires a message loop so that it can receive notifications from Windows and other processes. A message loop is the universal solution to the producer-consumer problem. With the operating system and other processes producing, the UI thread consuming.

A UI thread uses lots and lots of code that is not thread-safe. This includes major features implemented in COM, like drag+drop, the clipboard, the shell dialogs, ActiveX controls like a browser. And a raft of code that was never made thread-safe because the programmer didn't have to make it so, much easier to write. Those features require an STA thread, in other words a thread that initializes COM by passing COINIT_APARTMENTTHREADED to CoInitializeEx().

That is a promise to COM that the thread will be a good citizen, it is not allowed to make blocking calls and must pump a message loop. It is the message loop that COM uses to marshal a call from a worker thread to the STA thread in order to keep a COM object thread-safe. When all the calls are made from the same thread then there's never a safety issue. The underlying call is SendMessage(), with a ton of plumbing to copy the function arguments from one stack to another. CoInitializeEx() creates a hidden message window owned by the STA thread that processes the message and actually makes the call. Safely.