Why does Control.Invoke() calls PostMessage() instead of SendMessage()?

Question

Control.Invoke() calls PostMessage() and then waits until the UI thread finishes processing the message. So why it does not calls SendMessage() instead (which by default waits until the UI thread finishes processing the message).

Answer 1

Control.Invoke() is a dangerous method, many .NET programmers have deadlocked their program with it. It should be very strongly avoided because of this. Simple everyday operations like closing a window become perilous. You'll want to wait until a worker thread cannot invoke anymore since nothing good happens when the thread keeps running but the UI is gone. So you signal the thread with, say, AutoResetEvent and wait for it to complete.

Such a wait is very likely to deadlock your program when the thread is calling Invoke() at just the wrong time. The thread cannot complete because it is stuck in the Invoke() call, the UI thread cannot service it since it is stuck in the wait. A "deadly embrace", neither thread can make progress and your program will hang. Quite hard to debug since it is not predictable and doesn't happen often enough, only goes wrong when the thread calls Invoke at exactly the same time.

Breaking that deadlock requires knowing that an Invoke() call is in progress so it can be cancelled. It is not knowable when you use SendMessage(). The lock on which it blocks is hidden in the OS. I've recently posted an answer about the problems with SendMessage, everything you read there applies here as well.

So Microsoft did not implement it that way and they use PostMessage. They add an entry to the invoke queue, call PostMessage to wake up the UI thread so it goes looking through that queue. And specific to Invoke over BeginInvoke, they block on a ManualResetEvent in the queue entry, signaled when the UI thread completed the call to the delegate target.

Now they can do something to avoid the deadlock, when a window closes it looks through the invoke queue and cancels any that had that window as the invoke target. Or in other words, the lock that's invisible when you use SendMessage and causes deadlock now becomes visible and can be released to break the deadlock.