c# 怎么让刚才画的线消失

c# 如何让刚才画的线消失？
小弟C# 新手，用C# 划线时画出来了，那我如何才能让刚才画的线消失呢？代码如下
  private void Form1_Paint(object sender, PaintEventArgs e)
  {
  Graphics g = this.CreateGraphics();
  Pen p = new Pen(Color.Green, 1);
  g.DrawLine(p,20, 650, 20, 50);
  g.DrawLine(p, 20, 50, 1020, 50);
  g.DrawLine(p, 1020, 50, 1020, 650);
  g.DrawLine(p, 20, 650, 1020, 650);
  g.Dispose();
  p.Dispose();
  }

[解决办法]

C# code

Graphics g = this.CreateGraphics();                Pen p = new Pen(Color.Green, 1);                g.DrawLine(p, 20, 650, 20, 50);                g.DrawLine(p, 20, 50, 1020, 50);                g.DrawLine(p, 1020, 50, 1020, 650);                g.DrawLine(p, 20, 650, 1020, 650);                g.Clear(this.BackColor);//清除                g.Dispose();                p.Dispose();