private void button1_Click(object sender, EventArgs e)

{

//this代表当前窗体,在当前窗体上创建Graphics

//第一步:创建GDI对象

Graphics g = this.CreateGraphics();

//创建一个画笔

Pen p=new Pen (Color.Red,2);

//确定两个点

Point p1 = new Point(20, 20);

Point p2 = new Point(30, 40);

//调用GDI的DrawLine方法

g.DrawLine(p, p1, p2);

}