title: FindWindowEx is not running
body: <p>I have a problem with the API windows function FindWindowEx, in fact I obtain the handle of the MainWindow of the process, but when i try to retrieve the handle of one of its buttons with FindWindowEx, it's not running. 
  I had verified the window and its buttons with spy++, and everything runs well, even the handle of the mainwindow returned by my program matches the spy++'s one. i have tested the error code returned by "Marshal.GetLastWin32Error()",
   i always obtain error 1008. i have searched in many old posts dealing with my problem, but i hadn't find any solution for it. here is my code :</p>
  
  <pre><code>DllImport("user32.dll" , CharSet = CharSet.Auto)]
      public static extern IntPtr FindWindowEx(IntPtr parentHandle, IntPtr childAfter, string className, string windowTitle);
   // .... 
   IntPtr hwnd = proc.MainWindowHandle;
          string str = proc.MainWindowTitle;
          Console.WriteLine("Main window Title : " + str);
          Console.WriteLine("Main window Handle : " + hwnd.ToString());
          //Get a handle for the "suivant" button
          IntPtr hwndChild = FindWindowEx((IntPtr)hwnd, IntPtr.Zero, "Button", "suivant" );
          int error = Marshal.GetLastWin32Error() ; 
  </code></pre>