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    <p>My answer is very similar to @EricBrown's one, but there is one different point. </p> <p>Creating a <a href="http://blogs.msdn.com/b/nickkramer/archive/2006/05/06/591252.aspx" rel="nofollow noreferrer">nested message loop</a> with <code>MsgWaitForMultipleObjectsEx</code> may lead to code reentrancy on the same thread (via a window message dispatched by the inner <code>PeekMessage/TranslateMessage/DispatchMessage</code> pattern). At worst scenario, you may end up calling the same COM object method <em>before</em> the previous call has returned. </p> <p>I would first try using <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/ms680732%28v=vs.85%29.aspx" rel="nofollow noreferrer">CoWaitForMultipleHandles</a> with <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/ms693767%28v=vs.85%29.aspx" rel="nofollow noreferrer">COWAIT_DISPATCH_CALLS</a> (but without <code>COWAIT_DISPATCH_WINDOW_MESSAGES</code>). In case your COM object is provided by an out-of-proc server, this most likely should work. Otherwise, you should consider putting some reentrancy checks in place.</p> <p>I have a <a href="https://stackoverflow.com/questions/18050438/handling-ckeditor-async-calls-synchronously-with-c-sharp-winforms-webbrowser-con">related question</a> with some code showing how it could be done with C# (I had to use <code>COWAIT_DISPATCH_WINDOW_MESSAGES</code> there, otherwise the event I was after wasn't getting fired). </p> <p><strong>[UPDATE]</strong> Ideally, you should use <code>async/await</code> pattern for things like that and wrap your event as a task (e.g. <a href="https://stackoverflow.com/a/18317508/1768303">here's how</a>). I understand, sometimes it is not feasible to re-factor existing code to use this approach. However, if a pending operation takes considerable time to complete, a more user-friendly way to wait for its completion event might be just to show a modal dialog with a nice "please wait..." message (as discussed here in <a href="https://stackoverflow.com/a/18317508/1768303">comments</a>). You'd just close this dialog from your event handler. In fact, AFAIK, this is the only endorsed way for a WinForms app to enter a nested message loop. </p> <p><strong>[UPDATE]</strong> As Eric pointed out in comments, <code>COWAIT_DISPATCH_WINDOW_MESSAGES</code> is indeeded required for an STA thread. Apparently, <code>COWAIT_DISPATCH_CALLS</code> is intended for the new little-known <a href="http://msdn.microsoft.com/en-us/library/windows/apps/hh750290.aspx" rel="nofollow noreferrer">ASTA model</a> and <em><a href="http://msdn.microsoft.com/en-us/library/windows/desktop/ms693767%28v=vs.85%29.aspx" rel="nofollow noreferrer">has no meaning in other apartment types</a></em>. </p> <p>In case with out-of-proc COM servers, .NET event handlers are called back as free-threaded objects regardless of the waiting thread's apartment model (<a href="https://stackoverflow.com/q/18458398/1768303">in my experience</a>, it's never the same STA thread on which the out-of-proc object was originally created). Thus, waiting with <a href="http://msdn.microsoft.com/en-us/library/58195swd.aspx" rel="nofollow noreferrer">WaitHandle.WaitOne</a> (no pumping) should be sufficient. However, if the event handler accesses any state data besides the <code>WaitHandle</code>, proper synchronization is required (with <a href="http://msdn.microsoft.com/en-us/library/c5kehkcz.aspx" rel="nofollow noreferrer">locks</a> etc).</p>
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    1. COIf the code is in a STA, `COWAIT_DISPATCH_WINDOW_MESSAGES` is required. [Chris Brumme's article](http://blogs.msdn.com/b/cbrumme/archive/2004/02/02/66219.aspx) (referenced in your nested message loop article) emphasizes this. Incoming COM calls arrive via window messages. Overall, I prefer your solution (I just *knew* someone at MSFT had already packaged up the MsgWaitForMultipleObjects code); it's cleaner & simpler; but the odds are @Rauld will have to pump.
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    2. CO@EricBrown, a great point, thank you. I did not know `COWAIT_DISPATCH_CALLS` is not enough for STA. The article you mentioned looks a must-read, yet i overlooked it, get to catch-up.
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