title: Show a custom calendar dropdown with a derived DateTimePicker class
body: <p>My goal is to create a custom DateTimePicker class in .NET 2.0, which shows a custom calendar dropdown instead of the Windows default calendar popup.</p>
  
  <p>By observing Windows messages (see attached code), I am able to find and hide/close the calendar window after creation.</p>
  
  <p>However, a problem remains: After the calendar window is closed, something is still blocking the mouse input. For example, if you try to maximise the owner form of the customised DateTimePicker control after the calendar dropdown has been closed programmatically (attached code), the maximise button does not respond. Only the next click works. Interestingly, the "non-functional click" fires the DTN_CLOSEUP notification, so it appears that the WM_CLOSE did not properly close the calendar.</p>
  
  <p>Any hints on how to accomplish my task are highly appreciated :)</p>
  
  <pre><code>protected override void WndProc(ref System.Windows.Forms.Message m)
  {
      if (m.Msg == (int)SYSMSG.WM_REFLECT + (int)SYSMSG.WM_NOTIFY)
      {
          NMHDR nmhdr = (NMHDR)m.GetLParam(typeof(NMHDR));
          switch (nmhdr.code)
          {
              case DTN_DROPDOWN:
                  // Hide window
                  IntPtr calHandle = FindWindow("SysMonthCal32", null);
                  SendMessage(calHandle, (int)SYSMSG.WM_SIZE, 0, SP.Convert.MakeLong(0, 0));
  
                  this.BeginInvoke((MethodInvoker)delegate()
                  {
                      SendMessage(calHandle, (int)SYSMSG.WM_CLOSE, 0, 0);
                  });
                  break;
          }
  
      }
  
      base.WndProc(ref m);
  }
  </code></pre>