Questions about COM multithreading and STA / MTA

Question

Hi I am a beginner in COM. I want to test a COM dll in both STA and MTA modes. My first question is: is it possible a COM object supports both STA and MTA?

Now I imagine the STA code snippet below:

// this is the main thread
m_IFoo;
CoInitializeEx(STA); // initialize COM in main thread
CreateInstance(m_IFoo);
m_IFoo->Bar();

CreateThread(ThreadA);
// start ThreadA

// this is secondary thread
ThreadA()
{
    CoInitializeEx(STA);
    m_IFoo->Buz(); // call m_IFoo's method directly
}

Will this code work? Am I missing any fundamental things? I know the main thread needs a window message loop to let calls from other threads be executed. Do I have to do anything about it?

Now I move on to test MTA. If I merely replace "STA" with "MTA" in the above code, will it work？

Another question is: As a thread with GUI must be STA, I cannot initialize and test MTA in a GUI thread?

Thanks in advance and sorry for me being naive on COM and threading.

BrendanMcK BrendanMcK · Accepted Answer · 2011-06-06T22:06:47

Your code is not legal COM, because you are passing a pointer directly from one STA to another, which COM doesn't allow.

In COM, interface pointers have "apartment affinity", they can only be used within an apartment. To pass a pointer from one STA to another, or between STA and MTA, you have to 'marshal' the pointer to a safe representation, which is then unmarshaled by the receiving thread.

The simplest way to do this is using the Global Interface Table; you register the interface with it in one thread and get back a DWORD, which you then use in the other thread to get back a version of the interface that the other thread can use.

If both threads are MTA, you can avoid doing this. While STA are one-per-thread - each STA thread has its own aparment - the MTA is shared by all MTA threads. This means that MTA threads can pass COM pointers between themselves freely. (But they still need to marshal if passing pointers to or from STA threads.)

Generally speaking, you don't change code between STA or MTA, you usually decide this once at the outset. If the thread has UI, then it needs a message loop, and is usually STA. If there's no UI, you may decide to use MTA. But once you make that decision and write your code, it's rare to change to the other later, since picking one or the other has different requirements and assumptions that affect the code; change from STA to MTA or vice versa and you'd have to carefully review the code and see if things like pointer assignments needed to be changed.

Questions about COM multithreading and STA / MTA

2 Answers