Q:

send message to a window handle

I'm running an exe through which I get a handle of a control in another exe. Now what I want to do is send messages to the particular handle from my exe.

A:

You need to import the function using:
[DllImport("user32.dll")]
public static extern int SendMessage(
                  int hWnd,      // handle to destination window
                  uint Msg,       // message
                  long wParam,  // first message parameter
                  long lParam   // second message parameter
                  );

and define the message that you want to send like:
public const int <WM_YOURMESSAGE> = <yourvalue>; 

and then call SendMessage like any other function. 

