GDI基础

        private void button1_Click(object sender, EventArgs e)
        {
            Graphics g = this.CreateGraphics();//将本窗体创建为画板
            //Graphics不可以用New来实例化。
            Pen pen = new Pen(Color.Red,20);//创建一只笔，定义颜色和宽度

            g.DrawEllipse(pen ,100,100,200,200);//画园
            g.FillEllipse(Brushes.Blue,100,100,200,200);//填充颜色
        }

        private void button2_Click(object sender, EventArgs e)
        {
            Graphics g = this.CreateGraphics();
            Pen pen = new Pen(Color.Red,20);
            g.DrawRectangle(pen,20,20,600,20);
            g.FillRectangle(Brushes.Blue,20,20,600,20);
        }

        private void button3_Click(object sender, EventArgs e)
        {
            Graphics g = this.CreateGraphics();
            Pen pen = new Pen(Color.Red, 20);
            g.DrawLine(pen,60,90,500,90);
        }

        private void button4_Click(object sender, EventArgs e)
        {
            Graphics g = this.CreateGraphics();
            g.Clear(Form1.DefaultBackColor) ;//清空画板，更改为窗体默认的背景颜色
        }