C#之Graphics

         Graphics  g = this.CreateGraphics();

        private void button1_Click(object sender, EventArgs e)
        {
            g.DrawString("成功画线~！",this.Font,Brushes.Blue,this.button1.Location.X,button1.Location.Y+button1.Height+10);
        }

       //单击按钮清除刚才所画（还原到以前的图像）

        private void button2_Click(object sender, EventArgs e)
        {
           
        }

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请教一下，怎么实现button2_Click，不能使用Clear（）