SendMessage vs PostMessage + WaitForSingleObject

c++ winapi sendmessage postmessage waitforsingleobject
Filip Frącz picture Filip Frącz · Jul 21, 2011 · Viewed 11.8k times · Source

I was wondering what's the difference between calling SendMessage (which blocks) and calling PostMessage in conjunction with WaitForSingleObject. Thoughts?

Answer

Jörgen Sigvardsson picture Jörgen Sigvardsson · Jul 21, 2011

SendMessage() may be denied, if you call it from within the context of a COM call, where the COM object lives in an STA (calls are dispatched through the message pump). PostMessage() is not restricted to adhere to COM apartment rules.

Also, PostMessage() puts the message on the end of the window's message queue. SendMessage() bypasses the queue. You can find a lengthier discussion on message queues on Raymond Chen's blog The Old New Thing.

My point is that there is more to the difference between SendMessage() and PostMessage() than meets the eye. I really recommend going through Raymond's blog, as he has covered many gotchas over the years.

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