为要处理的panel添加绘画事件，代码写在panel的Paint事件中：

private void panel1_Paint(object sender, PaintEventArgs e)
        {

//（new 一个矩形，范围为panel的上半部分）

Rectangle rt1 = new Rectangle(0, 0, panel1.Width, (panel1.Height/2));

//（new一个渐变笔刷，渐变范围为上面新建的矩形，即panel的上半部分，渐变色从白色到蓝色，渐变角度LinearGradientMode.Vertical为垂直渐变）
            LinearGradientBrush br = new LinearGradientBrush(rt1, Color.White, Color.Blue,   LinearGradientMode.Vertical);
            Graphics g = e.Graphics;//为控件创建Graphics
            g.FillRectangle(br, rt1);//填充矩形

//(下面同上面的操作，渐变panel的下半部分)
            LinearGradientBrush br2 = new LinearGradientBrush(rt1, Color.Blue, Color.White, LinearGradientMode.Vertical);
            Rectangle rt2 = new Rectangle(0,(panel1.Height/2), panel1.Width, panel1.Height/2);
            g.FillRectangle(br2, rt2);

//实现圆角形状的panel
            List list = new List();//建立点集合list
            int width = this.panel1.Width;
            int height = this.panel1.Height;
            //panel左上的点
            list.Add(new Point(0, 4));
            list.Add(new Point(1, 4));
            list.Add(new Point(1, 3));
            list.Add(new Point(2, 2));
            list.Add(new Point(3, 1));
            list.Add(new Point(4, 1));
            list.Add(new Point(4, 0));
            //panel右上的点
            list.Add(new Point(width - 4, 0));
            list.Add(new Point(width - 4, 1));
            list.Add(new Point(width - 3, 1));
            list.Add(new Point(width - 2, 2));
            list.Add(new Point(width - 1, 3));
            list.Add(new Point(width - 1, 4));
            list.Add(new Point(width - 0, 4));
            //panel右下角的点
            list.Add(new Point(width - 0, height - 4));
            list.Add(new Point(width - 1, height - 4));
            list.Add(new Point(width - 1, height - 3));
            list.Add(new Point(width - 2, height - 2));
            list.Add(new Point(width - 3, height - 1));
            list.Add(new Point(width - 4, height - 1));
            list.Add(new Point(width - 4, height - 0));
            //panel左下角的点
            list.Add(new Point(4, height - 0));
            list.Add(new Point(4, height - 1));
            list.Add(new Point(3, height - 1));
            list.Add(new Point(2, height - 2));
            list.Add(new Point(1, height - 3));
            list.Add(new Point(1, height - 4));
            list.Add(new Point(0, height - 4));
            Point[] points = list.ToArray();//将以上圆角形状的点集合转换成点数组
            GraphicsPath shape = new GraphicsPath();//新建绘图路径对象
            shape.AddPolygon(points);将路径转换为多边形
            //将窗体的显示区域设为GraphicsPath的实例
            this.panel1.Region = new System.Drawing.Region(shape);

}