PostMessage for cross application messages.

Question

I'm trying to send a keystroke to another application. I can successfully find the window handle since using SendMessage worked exactly as intended.

However, when I switched the SendMessage over to PostMessage, the application no longer received the messages.

I did, however, find a workaround by using HWND_BROADCAST as the window handle, and it works fine, but isn't the ideal way to go about it.

What I'm asking is, I have a valid hWnd, how can I send it messages using PostMessage and not SendMessage?

Edit This is what I'm trying to do.

HWND Target = FindWindow(0, "Window Title Goes Here");
LPARAM lParam = (1 | (57<<16)); // OEM Code and Repeat for WM_KEYDOWN
WPARAM wParam = VK_SPACE;
PostMessage(HWND_BROADCAST, WM_KEYDOWN, wParam, lParam); // Works
PostMessage(Target, WM_KEYDOWN, wParam, lParam); // Doesn't Work
SendMessage(Target, WM_KEYDOWN, wParam, lParam); // Works, but I need Post

Answer 1

The PostMessage function does not work when the message numbers between 0 and WM_USER-1. Use RegisterWindowMessage function to register your own messages.

Answer 2

Sent messages and posted messages take completely different routeres. Target is recieving your posted message, it's just either filtering or dispatching it to another window. It gets to do what ever it wants with it. When you send the messages, it goes directly to the window procedure without filtering, so is most likely that cause of that issue.

I don't know why HWND_BROADCAST is working; my best guess is that a window other than Target is processing the message. Or maybe its even being sent to a different window than Target. (You do realize that HWND_BROADCAST sends the messages to every top level window)

There is a Win32 API function designed to send input, SendInput(), that places the messages on the input queue just like a user keypress. However this doesn't let you specify a window, it sends its input to the active window. To use it you would have to activate and switch focus to Target, which means the user would see that window move to the top (just like you Alt-Tabbed to it). Along that same route VBScript has a SendKeys() function that does the same thing, but is easier to use.

As a final alternative you could use SendMessageCallback() which will give you the behavior of an asynchronous SendMessage which is what I assume you want. (And is different than PostMessage. Posted messages go into the posted message queue, sent messages are delivered directly)