ASP.NET简单验证码

        PRotected void Page_Load(object sender, EventArgs e)        {            CreateCheckCodeImage(RndNum());        }        private string RndNum()        {            int number;            char code;            string checkCode = String.Empty;            System.Random random = new Random();            for (int i = 0; i < 4; i++)            {                number = random.Next();                if (number % 2 == 0)                    code = (char)('0' + (char)(number % 10));                else                    code = (char)('A' + (char)(number % 26));                checkCode += code.ToString();            }            Response.Cookies.Add(new HttpCookie("yzmcode", checkCode));            return checkCode;        }        private void CreateCheckCodeImage(string checkCode)        {            if (checkCode == null || checkCode.Trim() == String.Empty)                return;            System.Drawing.Bitmap image = new System.Drawing.Bitmap((int)Math.Ceiling((checkCode.Length * 12.5)), 22);            Graphics g = Graphics.FromImage(image);            try            {                //生成随机生成器                 Random random = new Random();                //清空图片背景色                 g.Clear(Color.White);                //画图片的背景噪音线                 for (int i = 0; i < 25; i++)                {                    int x1 = random.Next(image.Width);                    int x2 = random.Next(image.Width);                    int y1 = random.Next(image.Height);                    int y2 = random.Next(image.Height);                    g.DrawLine(new Pen(Color.Silver), x1, y1, x2, y2);                }                Font font = new System.Drawing.Font("Arial", 12, (System.Drawing.FontStyle.Bold | System.Drawing.FontStyle.Italic));                System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.DarkRed, 1.2f, true);                g.DrawString(checkCode, font, brush, 2, 2);                //画图片的前景噪音点                 for (int i = 0; i < 100; i++)                {                    int x = random.Next(image.Width);                    int y = random.Next(image.Height);                    image.SetPixel(x, y, Color.FromArgb(random.Next()));                }                //画图片的边框线                 g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);                System.IO.MemoryStream ms = new System.IO.MemoryStream();                image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);                Response.ClearContent();                Response.ContentType = "image/Gif";                Response.BinaryWrite(ms.ToArray());            }            finally            {                g.Dispose();                image.Dispose();            }        } 

 <img id="imgCode" width="83" height="38" src="ValiDateImage.aspx?<%=DateTime.Now %>" style="cursor: pointer;" alt="点击刷新" onclick="Show(this);">