Re: C++ COM STA model
George <George@discussions.microsoft.com> wrote:
Suppose we access function from one STA to another STA, the
marshelling is only responsible for argument pack/unpack and return
result pack/unpack?
It is also responsible for the call being executed on the correct
thread. Suppose you call from thread A to thread B - both STA. During
the call, thread A enters a modal message loop waiting for the call to
return. A message is posted to thread B's message queue. When thread B
dispatches this message, the method is called (in the context of thread
B, of course). Then, a message is posted to thread A's message queue, to
signal that the method has returned and results are available. When
thread A dispatches this message, it drops out of the modal message pump
and returns to the original caller.
Is marshelling used also for the function of synchronization control
purpose, like the hidden window message queue?
I don't understand this question. Synchronization between what and what?
Why marshell is needed even if we are using two STA inside the same
process?
Because an apartment-threaded COM object is not thread-safe. All calls
to it must be done on the same thread that created it in the first
place. So when you call it from another thread - STA or otherwise - the
call must be trasferred, or in other words marshalled, to the object's
"native" thread.
--
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