程序代码：

           private void OnFormPaint(object sender, System.Windows.Forms.PaintEventArgs e)
           {
            Graphics g = e.Graphics;

            Pen myPen = new Pen(Color.Blue, 1); //创建一个自定义画笔对象

            //创建一个正方形rect
            int myRect_X = this.ClientRectangle.Right / 6;
            int myRect_Y = this.ClientRectangle.Bottom / 6;
            int myRect_Width;
            int myRect_Height;
            if (this.ClientRectangle.Height < this.ClientRectangle.Width)
            {

                myRect_Width = this.ClientRectangle.Height *2/ 3;
                myRect_Height = this.ClientRectangle.Height*2 / 3;
            }
            else
            {
                myRect_Width = this.ClientRectangle.Width / 2;
                myRect_Height = this.ClientRectangle.Width / 2;
            }


            Rectangle rect = new Rectangle(myRect_X, myRect_Y, myRect_Width, myRect_Height);

            g.DrawEllipse(myPen, rect); //画出一个内切于矩形rect的圆


            g.TranslateTransform(ClientSize.Width / 2, ClientSize.Height / 2, MatrixOrder.Append);

            DrawClock(e.Graphics);
        }

        private void DrawClock(Graphics g)
        {
            const double pai = Math.PI;
            Point center = new Point(0, 0);

            DateTime time = DateTime.Now;
            //读取时间；
            double secAng = 2.0 * pai * time.Second / 60.0;
            double minAng = 2.0 * pai * (time.Minute + time.Second / 60.0) / 60.0;
            double hourAng = 2.0 * pai * (time.Hour + time.Minute / 60.0) / 12.0;
            //各指针单位换算；
            int r = Math.Min(ClientSize.Width, ClientSize.Height) / 2;
            int secHandLength = (int)(0.6 * r);
            int minHandLength = (int)(0.4 * r);
            int hourHandLength = (int)(0.2 * r);
            //指针的长度定义；

            Point secHand = new Point((int)(secHandLength * Math.Sin(secAng)),
             (int)(-secHandLength * Math.Cos(secAng)));
            Point minHand = new Point((int)(minHandLength * Math.Sin(minAng)),
             (int)(-minHandLength * Math.Cos(minAng)));
            Point hourHand = new Point((int)(hourHandLength * Math.Sin(hourAng)),
             (int)(-hourHandLength * Math.Cos(hourAng)));
            //刷新指针；

            Pen SecPen = new Pen(Color.Red, 1);
            g.DrawLine(SecPen, center, secHand);

            Pen MinPen = new Pen(Color.RoyalBlue, 3);
            g.DrawLine(MinPen, center, minHand);

            Pen HourPen = new Pen(Color.CornflowerBlue, 5);
            g.DrawLine(HourPen, center, hourHand);
            //指针的样式定义；
        }