How can you tell if SendMessage() was successful (message was delivered)?

Question

is it possible to determine if a SendMessage() call was successful and delivered my message to the target window?

The SendMessage() description in the Windows API seems

Answer 1

There is no way to reliably tell, whether a call to SendMessage succeeded or failed. After all, it only has a single return value, and the entire range of values is used. There is no designated error value.

Playing tricks with the calling thread's last error code won't work either. The scheme proposed in this answer (clearing the last error code prior to the call and evaluating it after the call) is brittle. It's easy to see that it can fail, when sending a message to a window owned by the calling thread (as that window's window procedure is at liberty to change the last error any way it sees fit).

It's not quite as well known, that this can also fail when sending a message to a window owned by a different thread. The code suggests, that no intervening API call can happen in the calling thread. Yet in that scenario, SendMessage will also dispatch incoming cross-thread messages (see When can a thread receive window messages?), allowing the thread's last error code to change.

The only option then is to use SendMessageTimeoutW, as that API reports both the result of the call, as well as an error/success indicator. Passing the SMTO_NOTIMEOUTIFNOTHUNG flag makes sure that an arbitrarily chosen timeout will not adversely affect the outcome.