C++/C# interop causes OleInitialize (STA) to fail?

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Guest

Is there something about C++ / C# interop that initializes the threading
model to MTA so that OleInitialize will fail?

I have a mostly C++ app that calls a single C# class DLL. Only one source
file in the C++ app is compiled as managed (with /clr) – the single file in
which only one of the many functions creates a class object from the C# DLL
and invokes its methods (to decode an XML file the easy way).

During normal execution my code eventually creates an instance of an
MSFlexGrid ocx object (a grid control with column and row headers), which in
turn calls AfxOleInit and therefore OleInitialize. Works fine in normal
execution. I believe OleInitialize is by default STA.

By “normal†execution I mean the situation when I do not execute any code in
that single C++ file that calls into the C# DLL. If, however, I call
functions in that C++ file (even if they do not call the only function that
invokes C#), then when I create the MSFlexGrid object the internal call to
OleInitialize fails with the Trace message “Warning: OleInitialize returned
scode = RPC_E_CHANGED_MODE ($80010106)â€.

From the help files it sounds like OleInitialize expects to be run in STA
mode, and the error message means “A previous call to CoInitializeEx
specified the concurrency model for this apartment as multithread apartment
(MTA)â€. I tried putting breakpoints on every bit of MFC / ATL source code
that calls OleInitialize or CoInitialize to see if I could find the offender,
but nothing trapped.

If I create the MSFlexGrid before invoking C#, all is fine, and if I call
OleInitialize at the very beginning of my app all is fine. But I’m worried
there is a threading disaster waiting to happen.

What’s the deal?
 
That's roughly what I tried, and it does indeed solve the problem to call
CoInitialize (or OleInitialize) before the Framework calls it with
COINIT_MULTITHREADED.

Two questions:
1) My original question: is this a threading disaster waiting to happen by
setting the mode to STA? Are there other conflicts I should be aware of?

2) By setting the mode to STA will that disable the garbage collector?

Thanks!
 
Hi,
That's roughly what I tried, and it does indeed solve the problem to call
CoInitialize (or OleInitialize) before the Framework calls it with
COINIT_MULTITHREADED.

Two questions:
1) My original question: is this a threading disaster waiting to happen by
setting the mode to STA? Are there other conflicts I should be aware of?
Click to expand...

Definitely not. The runtime simply defaults to MTA, if it
detects that CoInitialize was not called before, and
if the main entry point of an executable assembly doesn't
apply the STAThreadAttribute.
 
Bill,
Is there something about C++ / C# interop that initializes the threading
model to MTA so that OleInitialize will fail?
Click to expand...

Well, by default the CLR will initialize managed threads into the MTA. In C#
or VB, you can usually prevent this for the application's main thread by
tagging the entry point method with the [STAThread] attribute.
Unfortunately, this doesn't work reliably in Managed C++ because the main()
function is not actually the application's entry point (the actual one is in
the CRT).

Sounds like this might help:
http://support.microsoft.com/kb/824480
 
While the article applies directly to the issue I have, the proposed
solution is more appropriate for a command line app since you can more easily
get at the startup routine. But the article accurately describes my situation.

I merely added a call to OleInitialize(NULL) very early on in my app before
any calls to the Framework (which initializes COM as MTA), and that seems to
have solved my problem.

Thanks for your help!
 
Bill,
While the article applies directly to the issue I have, the proposed
solution is more appropriate for a command line app since you can more
easily
get at the startup routine. But the article accurately describes my
situation.

I merely added a call to OleInitialize(NULL) very early on in my app
before
any calls to the Framework (which initializes COM as MTA), and that seems
to
have solved my problem.
Click to expand...

Glad to know. If your app does not have a managed entry point, then that
probably works. The problem with this, if there's a managed entry point, is
that you're not guaranteed it will work because you can't predict what the
runtime did before you...
 
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