Thread: The return value from WndProc

Originally Posted by megafiddle

What qualifies as processing a message?

I would assume it's more than just adding a case for it in the switch?

Code:

case WM_LBUTTONDOWN :
    return 0;

Yes, that is you doing something with the message... in this case you are preventing it from being passed on to the DefWindowProc(). (A common use is with WM_ERASEBACKGROUND to prevent flickering on the screen)


But would this be considered processing:

Code:

case WM_LBUTTONDOWN :
    counter++;
    return 0;

Why would the caller to WndProc (the operating system?) be interested in that example?

The thing is that it's sending you the message and you are not passing it on to DefWindowProc(), and returning 0, so as far as the OS is concerned, you've done something with the message so it won't process it any further. If you returned 1 instead of 0 it will continue to process the message after the switch returns.