Re: COM calls reentering in STA

"Igor Tandetnik" <>
Thu, 9 Jul 2009 14:40:46 -0400
Scott McPhillips [MVP] <org-dot-mvps-at-scottmcp> wrote:

I wrote an exe COM server and its client, which is an Office COM
addin. All unmanaged using ATL. Client and server use STA. The client
initially passes in a callback pointer the server will use to call
its methods. The server is multithreaded so it uses the GIT table to
marshall calls to the client.

STA and multithreaded is an unusual combination, especially for a
server. Are you creating multiple threads, each entering STA? Why don't
you just have them all join MTA? Anyway, that's irrelevant to your main

The problem I have it that calls from
multiple server threads, made to the same client object and method,
reenter in the client instead of waiting until the first call has

Your client methods yield - meaning, do something that causes window
messages to be processed. E.g. this happens if you show a modal dialog
box or a context menu, or make an outgoing cross-apartment COM call.
Incoming cross-apartment COM calls are delivered to an STA apartment in
the form of a custom window message; yielding allows a nested reentrant
call to come in.

See also IMessageFilter.

So I wonder if this is expected behavior.

Yes - and a royal pain to deal with.

The idea that using the
marshalled pointer switches threads so the calls all originate from
one thread may be true behind the scenes

Actually, it doesn't, they don't, and it isn't. None of your many STA
threads on the server is any more special than others. Another way to
say it: marshalling never results in a proxy to a proxy - you always get
a direct proxy to the original object.

but does not seem to be
enough to provide serialization of the calls. Why does the main
thread not wait for the COM call to return before resuming execution?

Which thread is the "main" one?

If COM does not serialize the calls, what do I have to do to
serialize them?

One way would be not to yield in your callback method.
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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