Re: ATL threading problem using client COM interface

"Igor Tandetnik" <>
Sun, 24 Aug 2008 10:46:21 -0400
"Edward Diener" <> wrote in message

Igor Tandetnik wrote:

"Edward Diener" <> wrote in
message news:O90YIBZBJHA.5468@TK2MSFTNGP04.phx.gbl

So what you are saying is that in my COM server, when I want to
pass a client interface pointer from the main thread in my STA to
my worker thread, I need to use one of the techniques above ?

Yes. You would also need to initialize COM on your worker thread - it
looks like CWorkerThread doesn't itself do that.

My COM Server is a DLL. It does not appear that initializing COM in a
DLL is correct. But perhaps I am wrong about this.

You are wrong about this. Every thread that wants to do COM work needs
to initialize COM first. It doesn't matter whether or not the
CreateThread call that created this thread is part of a DLL.

Now, if you have an in-proc (DLL) COM server, then the thread your
objects are created on must necessarily have already initialized COM -
otherwise, the client wouldn't be able to call CoCreateInstance and
create your object in the first place. So you shouldn't initialize COM
on client-created threads.

But if you create your own threads, there's no magic fairy that would
somehow initialize COM on them. It's your responsibility.

Which part of "CComGITPtr" did you find unclear?

That class deals with the global interface table.

"Global" here really means "global within a process".

If the interface is
created by the client and passed to me on the server is it valid to
add that interface to the GIT so that my different server threads can
use it in place of the marshalling functions you also mentioned ?

Yes. Note that the act of retrieving a pointer from GIT _is_
marshalling. It's not like you eliminate it somehow.

I also realized, if I am not mistaken, that I could use free threading
rather than apartment threading

You could. It essentially achieves the same effect: right at the time
when the client calls CoCreateInstance to get an instance of your
component, COM spins a worker thread that enters MTA, and creates your
component there. Your interface pointer is marshalled to the client, and
any calls the client makes on your object are marshalled back to MTA
threads. If the client passes its own interfrace pointers as parameters
to those methods, they are also automatically marshalled at that time.

Note that, when you created UI from your STA component, you could rely
on the client's message pump to deliver messages to your windows. Once
you have an MTA component, you are running in a different threads from
your client. If you want to display UI, you would have to spawn a
dedicated thread for it and have it run a message pump.
With best wishes,
    Igor Tandetnik

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