Control that simulates dragging of the window title bar

Question

I've built a custom control, and I'd like to allow people to click and drag on my control just as if they were dragging on the window title bar. What is the best way to do this?

So far I've been unsuccessful at leveraging the mouse down, up, and move events to decipher when the window needs to be moved.

Answer 1

The most effective way to do this is to handle the WM_NCHITTEST notification.

Override the form's WndProc method and add the following code:

if (m.Msg == 0x0084) {              //WM_NCHITTEST
    var point = new Point((int)m.LParam);
    if(someRect.Contains(PointToClient(point))
        m.Result = new IntPtr(2);   //HT_CAPTION
}

However, I don't think the message will be sent if there is a control at that point.

Answer 2

If you want to make a part of the form behave like the caption, the WM_NCHITTEST trick that SLaks gave is the way to go. But if you want to make a child window be able to drag the form and there is another way.

Basically if you send a WM_SYSCOMMAND message to DefWindowProc with the MOUSE_MOVE command id, then Windows will go into drag mode. This basically how the caption does it, but by cutting out the middle man, we can initiate this drag from a child window, and we don't get all of the other caption behaviors.

public class form1 : Form 
{
  ...

  [DllImport("user32.dll")]
  static extern IntPtr DefWindowProc(IntPtr hWnd, uint uMsg, UIntPtr wParam, IntPtr lParam);
  [DllImport("user32.dll")]
  static extern bool ReleaseCapture(IntPtr hwnd);

  const uint WM_SYSCOMMAND = 0x112;
  const uint MOUSE_MOVE = 0xF012;      

  public void DragMe()
  {
     DefWindowProc(this.Handle, WM_SYSCOMMAND, (UIntPtr)MOUSE_MOVE, IntPtr.Zero);
  }


  private void button1_MouseDown(object sender, MouseEventArgs e)
  {
     Control ctl = sender as Control;

     // when we get a buttondown message from a button control
     // the button has capture, we need to release capture so
     // or DragMe() won't work.
     ReleaseCapture(ctl.Handle);

     this.DragMe(); // put the form into mousedrag mode.
  }