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 = ; i < ; i++)

            {

                number = random.Next();

                if (number %  == )

                    code = (char)('' + (char)(number % ));

                else

                    code = (char)('A' + (char)(number % ));

                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)), );

            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 = ; i < ; 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", , (System.Drawing.FontStyle.Bold | System.Drawing.FontStyle.Italic));

                System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(, , image.Width, image.Height), Color.Blue, Color.DarkRed, 1.2f, true);

                g.DrawString(checkCode, font, brush, , );

                //画图片的前景噪音点

                for (int i = ; i < ; 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), , , image.Width - , image.Height - );

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