c# scroll panel with picturebox by dragging mouse

private Point _StartPoint;
void pictureBox1_MouseDown(object sender, MouseEventArgs e) {
  if (e.Button == MouseButtons.Left)
    _StartPoint = e.Location;
}
void pictureBox1_MouseMove(object sender, MouseEventArgs e) {
  if (e.Button == MouseButtons.Left) {
    Point changePoint = new Point(e.Location.X - _StartPoint.X, 
                                  e.Location.Y - _StartPoint.Y);
    panel1.AutoScrollPosition = new Point(-panel1.AutoScrollPosition.X - changePoint.X,
                                          -panel1.AutoScrollPosition.Y - changePoint.Y);
  }
}