Re: Regarding threading appartment in COM..

"Igor Tandetnik" <>
Sat, 27 May 2006 12:39:13 -0400
"Jigar Mehta" <> wrote in message

So, as I understood, if i have out of process, and MTA, pool of
threads will be created (do we know number or can we configure this

No and no, as far as I can tell. It's a dynamic pool, it grows and
shrinks depending on load.

and one of them will be chosen for method calls from all the
objects created for that COM Class.

Not quite. A free thread from the pool is independently chosen for every
method call. There's no guarantee that the same object will always be
called on the same thread, let alone that all objects from the same
coclass will be called on the same thread. Indeed, a single instance may
be called on multiple threads at the same time.

But what will be the scenario if I have in-process DLL with MTA (free)
applied (where proxy-stub is not required)

It is incorrect to state that proxy/stub is never required for in-proc
server. It is, when a call crosses apartment boundaries.

When a client thread that entered MTA creates your MTA in-proc object,
it gets a direct pointer. A call through this pointer just goes directly
to your method, on the same thread. Of course the client may create
multiple MTA threads, freely share your object's pointer between them,
and call you simultaneously on these threads.

When a client thread that entered STA creates your MTA in-proc object, a
pool of MTA threads is created by COM in the client's processes (just
like in out-of-proc case), your object is created on one of these MTA
threads, and the client gets a proxy. A call through this proxy selects
a thread from MTA pool and runs your method on this thread.

there also a pool of
threads will be created and my method call will be serviced by any of
those threads ?

Sometimes, yes.

And I was really confused by reading the statements from books that
You need to call CoInitializeEx(...APARTMENT...) to move your thread
to the MTA.

No, you need to call CoInitializeEx(COINIT_MULTITHREADED). Either your
book is wrong, or you are misreading or misremembering it.

Now, what happens here internally ?

COM initializes its internal data structures, and marks the thread as
belonging to MTA. Why do you care?

where is my thread
moved ?

Nothing is physically moved anywhere, just some COM internal data
structures are updated.

is my thread's ID entered symbolically somewhere in that
MTA's memory area so that calls from my threads are recognized
properly ?

Very likely.

Please clear me. What happens to my thread when I call
coinitia.... with apartment option, how will COM architecture know
that my thread is also now in MTA.. ??

When you call CoInitialize or CoInitializeEx(COINIT_APARTMENTTHREADED)
or OleInitialize, your thread enters STA, not MTA. A thread cannot be in
both MTA and STA simultaneoulsy.
With best wishes,
    Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925

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