a.cs

private const int   WM_NCHITTEST  = 0x84;
private const int   HTCAPTION     = 2;
private const int   HTLEFT        = 10,
                    HTRIGHT       = 11,
                    HTTOP         = 12,
                    HTTOPLEFT     = 13,
                    HTTOPRIGHT    = 14,
                    HTBOTTOM      = 15,
                    HTBOTTOMLEFT  = 16,
                    HTBOTTOMRIGHT = 17;
 
private const int   RESIZE_BORDER   = 4;
private const int   TITLEBAR_HEIGHT = 32;
 
protected override void WndProc( ref Message msg )
{
  switch( msg.Msg )
  {
    case WM_NCHITTEST:
    {
      // Get the cursor position
      var cursor = new Point( msg.LParam.ToInt32() );
 
      cursor = PointToClient( cursor );
 
      // Check for border resizing
      var result = 0;
 
      if( cursor.X <= RESIZE_BORDER )
      {
        // Leftmost
        if( cursor.Y <= RESIZE_BORDER )
        {
          result = HTTOPLEFT;
        }
        else if( cursor.Y >= ClientRectangle.Height - RESIZE_BORDER )
        {
          result = HTBOTTOMLEFT;
        }
        else
        {
          result = HTLEFT;
        }
      }
      else if( cursor.X >= ClientRectangle.Width - RESIZE_BORDER )
      {
        // Rightmost
        if( cursor.Y <= RESIZE_BORDER )
        {
          result = HTTOPRIGHT;
        }
        else if( cursor.Y >= ClientRectangle.Height - RESIZE_BORDER )
        {
          result = HTBOTTOMRIGHT;
        }
        else
        {
          result = HTRIGHT;
        }
      }
      else
      {
        // Top and bottom edges left
        if( cursor.Y <= RESIZE_BORDER )
        {
          result = HTTOP;
        }
        else if( cursor.Y >= ClientRectangle.Height - RESIZE_BORDER )
        {
          result = HTBOTTOM;
        }
      }
 
      // There was a change, return the border hit
      if( result != 0 )
      {
        msg.Result = (IntPtr)result;
        return;
      }
 
      // If it is on the title bar, let the system know to allow the user to drag the form around
      if( cursor.Y < TITLEBAR_HEIGHT )
      {
        msg.Result = (IntPtr)HTCAPTION;
        return;
      }
 
      break;
    }
  }
 
  base.WndProc( ref msg );
}