程序代码：

        private void timer1_Tick(object sender, EventArgs e)
        {
            for (int i = 210; i <= 850; i += 1)
            {
                for (int j = 320; j <= 500; j += 1)
                {
                    Point ap = new Point(i, j);
                    IntPtr ahdc = GetDC(new IntPtr(0));
                    int ac = GetPixel(ahdc, ap);
                    if (ac == 255)
                    {
                        label1.text = "Error"
                        break;
                    }
                }
            }
        }