动态生成验证码

private string GenerateCheckCode()
        {
            int number;
            string strCode = string.Empty;
            //随机种子
            //Random random = new Random();
            //for (int i = 0; i < 4; i++) //验证码长度为4
            //{
            //    //随机整数
            //    number = random.Next();
            //    //字符从0-9，A-Z中产生，对应的ASCII码为48-57，65-90
            //    number = number % 36;
            //    if (number < 10)
            //    {
            //        number += 48;
            //    }
            //    else
            //    {
            //        number += 55;
            //    }
            //    strCode += ((char)number).ToString();
            //}

              //产生随机字符串
            strCode = GenCode(4);

            //将字符串保存在Cookies
            Response.Cookies.Add(new HttpCookie("gst_checkCode", strCode));
            return strCode;
        }

        private string GenCode(int num)
        {
            string[] source ={"0","1","2","3","4","5","6","7","8","9",
                                "A","B","C","D","E","F","G","H","i","J","K","L","M","N",
                                "o","P","Q","R","S","T","U","V","W","X","Y","Z"};
            string code = "";
            Random rd = new Random();
            for (int i = 0; i < num; i++)
            {
                code += source[rd.Next(0, source.Length)];
            }
            return code;
        }

        //生成图片
        private void CreateCheckCodeImage(string checkCode)
        {
            //若检验码为空，则直接返回
            if (checkCode == null || checkCode.Trim() == string.Empty)
                return;
            //根据验证码长度确定输出图片的长度
            System.Drawing.Bitmap image = new Bitmap((int)Math.Ceiling(float.Parse((checkCode.Length * 15).ToString())), 20);

            //创建Graphics对象
            System.Drawing.Graphics g = Graphics.FromImage(image);

            //生成随机数种子
            Random random = new Random();
            //清除图片背景颜色
            g.Clear(Color.White);
            //画背景图片的噪音线10条
            for (int i = 0; i < 6; i++)
            {
                //噪音线起点坐标(x1,y1),终点坐标(x2,y2)
                int x1 = random.Next(image.Width);
                int x2 = random.Next(image.Width);
                int y1 = random.Next(image.Height);
                int y2 = random.Next(image.Height);
                //用银色线画出噪音线
                g.DrawLine(new Pen(Color.Silver), x1, y1, x2, y2);
            }
            //输出图片中校验码的字体:12号Arial,粗斜体
            Font font = new Font("Arial", 12, FontStyle.Bold);
            //线性渐变画刷
            System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Width), Color.Blue, Color.Purple, 1.2f, true);
            g.DrawString(checkCode, font, brush, 2, 2);
            //画图片的前景噪点50个
            for (int i = 0; i < 50; i++)
            {
                int x = random.Next(image.Width);
                int y = random.Next(image.Height);
                image.SetPixel(x, y, Color.FromArgb(random.Next()));
            }
            //画图片段边框线
            g.DrawRectangle(new Pen(Color.SaddleBrown), 0, 0, image.Width - 1, image.Height - 1);
            //创建内存流用于输出图片
            System.IO.MemoryStream ms = new System.IO.MemoryStream();
            //图片格式指定为png
            image.Save(ms, System.Drawing.Imaging.ImageFormat.Png);

            //清除缓冲区流中的所有输出
            Response.ClearContent();
            //输出流的Http Mime类型设置为"image/Png"
            Response.ContentType = "image/Png";
            //输出图片的二进制流
            Response.BinaryWrite(ms.ToArray());

            //释放Bitmap和Graphics对象
            g.Dispose();
            image.Dispose();
        }