Point srcPoint;
    private void panel1_MouseMove(object sender, MouseEventArgs e)
    {
      if ((e.Button & MouseButtons.Left) == MouseButtons.Left)
      {
        if (srcPoint.IsEmpty)
        {
          srcPoint = e.Location;
        }

        Point desPoint = e.Location;

        Control ctl = sender as Control;

        if (ctl != null)
        {
          Pen pen = new Pen(Brushes.Black, 2.0F);
          using (Graphics g = ctl.CreateGraphics())
          {
            g.DrawLine(pen, srcPoint, desPoint);
          }

        }

        srcPoint = desPoint;
      }
    }