SendMessage vs PostMessage + WaitForSingleObject

Question

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

Answer 1

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.

Answer 2

PostMessage and WaitForSingleObject allow you to do asynchronous messaging. You can send a message, do other things, and check back for a reply later. SendMessage is synchronous and requires you to wait.

Answer 3

I've always thought that SendMessage calls your windows procedure directly, skipping the message queue; while PostMessage just adds the message to the queue.

Answer 4

SendMessage is a single API call, therefore less prone to your errors. Go with the built-in rather than rolling your own.