Question

I have own control and I need to resize runtime by dragging. To resize bottom and right borders I use this:

protected override void OnMouseDown(MouseEventArgs e)
{
  SL = new System.Drawing.Point(Location.X + e.Location.X, Location.Y + e.Location.Y);
  SP = new System.Drawing.Point(Location.X, Location.Y);

  if (e.X <= m)
    _OnLeft = true;

  if (e.X >= Width - m)
    _OnRight = true;

  if (e.Y <= m)
    _OnTop = true;

  if (e.Y >= Height - m)
    _OnBottom = true;
}

protected override void OnMouseMove(MouseEventArgs e)
{
  // Change Width - right
  if (_OnRight && (!_OnTop && !_OnBottom))
  {
    if (e.X <= 1)
      return;
    Width = e.X;
    return;
  }

  // Change Height - bottom
  if (_OnBottom && (!_OnLeft && !_OnRight))
  {
    if (e.Y <= 1)
      return;
    Height = e.Y;
    return;
  }
}

All works fine. But I have problems with Top and Left resizing:

// Change Width - left
if (_OnLeft && (!_OnTop && !_OnBottom))
{
  // Problem part - I don't know condition to return
  if (Width + Left - e.X <= 1)
    return;
  Left += e.X - SL.X + SP.X;
  // How to get right width
  Width += Left - e.X;
  return;
}

// Change Height - top
if (_OnTop && (!_OnLeft && !_OnRight))
{
  // Problem part - I don't know condition to return
  if (Height + Top - e.Y <= 1)
    return;
  Top += e.Y - SL.Y + SP.Y;
  // How to get right height 
  Height += Top - e.Y;
  return;
}

Something like that. Have ideas?

Answer 1

Pretty much the only way to cleanly allow .NET control resizing is to use P/Invoke. This exact code is not tested, but I have used this resizing method many times so it should work:

First, the P/Invoke external declarations:

private static class UnsafeNativeMethods
{
    [DllImport("user32.dll")]
    public static extern bool ReleaseCapture();
    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    public static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, IntPtr lParam);
}

Next, call the P/Invoke functions to have the operating system handle the resize:

protected override void OnMouseDown(MouseEventArgs e)
{
    int msg = -1; //if (msg == -1) at the end of this, then the mousedown is not a drag.

    if (e.Y < 8)
    {
        msg = 12; //Top
        if (e.X < 25) msg = 13; //Top Left
        if (e.X > Width - 25) msg = 14; //Top Right
    }
    else if (e.X < 8)
    {
        msg = 10; //Left
        if (e.Y < 17) msg = 13;
        if (e.Y > Height - 17) msg = 16;
    }
    else if (e.Y > Height - 9)
    {
        msg = 15; //Bottom
        if (e.X < 25) msg = 16;
        if (e.X > Width - 25) msg = 17;
    }
    else if (e.X > Width - 9)
    {
        msg = 11; //Right
        if (e.Y < 17) msg = 14;
        if (e.Y > Height - 17) msg = 17;
    }

    if (msg != -1)
    {
        UnsafeNativeMethods.ReleaseCapture(); //Release current mouse capture
        UnsafeNativeMethods.SendMessage(Handle, 0xA1, new IntPtr(msg), IntPtr.Zero);
        //Tell the OS that you want to drag the window.
    }
}

Finally, override OnMouseMove to change the cursor based on where it is on the control. I’ll leave that part to you, because it’s almost the same code as the previous snippet.