Making an if in win32

How do I make and if in win32? Like if the Right Click is press, it draws a line? This is what I tried so far. Without the ! before the command it will draw the line without me clicking and with ! it never draws it.

 if(!WM_RBUTTONDOWN){
        MoveToEx(hdc, 5, 10, NULL);
        LineTo(hdc, 400, 300);
        }