c# 如何让刚才画的线消失？

Question

 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();
       }

Answer 1

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();

Answer 2

        int a = 0;
        private void button3_Click(object sender, EventArgs e)
        {
            a++;
            this.Refresh();
        }        private void Form1_Paint(object sender, PaintEventArgs e)
        {
            if (a%2==0)
            {
                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();
            }
        }

Answer 3

g.Clear(this.BackColor);//清除
1楼正解