Re: Simple COM EXE Server question

"Igor Tandetnik" <>
Thu, 14 Feb 2008 12:41:05 -0500
Andy Larter <> wrote:

Please can you confirm that my understanding of COM out-of-process
servers is correct.

1) The threading model of all objects in a COM EXE is determined by

Not directly. The threading model is determined by whether the thread
that registers the class factory for the object has entered STA or MTA.
I'm talking about the thread that calls CoRegisterClassObject (possibly
indirectly via CAtlExeModuleT::RegisterClassObjects). Typically, but not
necessarily, that's the application's main thread.

Now, if you use CAtlExeModule in your application (as you would in a
default wizard-generated project), it initializes COM in its constructor
based on those macros, and registers class factories in its Run()
method. So if you want to play funny games with apartments, you can't
use CAtlExeModule and will have to roll equivalent functionality

CComObjectRoot uses thread-safe reference counting. For your own
objects, you can derive from CComObjectRootEx instead and specify
explicitly whether or not you want it to be thread-safe. But you can't
do this for class factories (unless you are willing to write a custom
one), so their thread-safety is controled by those macros. Naturally, if
you want the object to live in MTA, it has to be thread-safe. If the
object lives in STA, you can still make it thread-safe, it'll just add
slight performance overhead.

2) You can't mix the threading model of objects in a COM EXE, they are
either all apartment or all free threaded, depending on the macro.

In principle, you can. You just register class factories for different
CLSIDs from different apartments. But default ATL project setup is not
designed for this, so it will take some work to suppress default
behavior and put in custom one.

In addition, it is possible to write a fancy class factory that, while
itself registered on one apartment, actually creates objects in other
apartments. CComClassFactoryAutoThread does this.

3) The only way to use say a apartment threaded object in a free
threaded server is to host the object in a COM DLL.

Are you talking about objects the server itself implements? In addition
to techniques described in my response to question #2, you can create
your own object on an STA thread (which presumably you create
specifically for this purpose) with CComObject::CreateInstance,
completely bypassing COM registration and class factory mechanism. Don't
forget to marshal the object's interface pointer to your MTA thread, and
don't forget to run a message pump on your STA thread.
With best wishes,
    Igor Tandetnik

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