SendMessage WM_LBUTTONDOWN/UP works on buttons but not window

I am trying to send some simple mouse down/up messages to Windows Calculator using SendMessage. I have been able to press the buttons by sending the messages to the buttons directly. However, I have not been able to successfully send the same messages to the main calculator window handle. Given that hWnd is the window handle to calculator, this is my c开发者_如何学JAVAode.

IntPtr fiveKey = FindWindowEx(hWnd, IntPtr.Zero, "Button", "5"); 

int x = 5; // X coordinate of the click
int y = 5; // Y coordinate of the click

IntPtr lParam = (IntPtr)((y << 16) | x); // The coordinates
IntPtr wParam = IntPtr.Zero; // Additional parameters for the click (e.g. Ctrl)
const uint downCode = 0x201; // Left click down code
const uint upCode = 0x202; // Left click up code
SendMessage(fiveKey, downCode, wParam, lParam); // Mouse button down
SendMessage(fiveKey, upCode, wParam, lParam); // Mouse button up

Can anyone explain to me why sending the messages to hWnd instead of fiveKey with the x/y offsets changed to the position of the "5" key does not work? I would like to eventually use this code to simulate mouse clicks on a different application that doesn't have buttons like calculator.

I'm not sure I follow you. Are you trying to send WM_LBUTTONDOWN to the main window with the coordinates of where the 5 button is, with the hopes that the 5 button will get "clicked"? If so, that's just not going to work. WM_LBUTTONDOWN is only ever sent to the window under the mouse cursor. In theory the main window could handle WM_LBUTTONDOWN and see if any of its child windows are at that location, but nobody does that because that's not how WM_LBUTTONDOWN is designed to work.