<img src="../Common/WebVcode.aspx" title="看不清？点此更换" alt="看不清？点此更换" onclick="this.src=‘/Common/WebVcode.aspx?a=‘+Math.random();" />    


     protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                Session["Vcode"] = CreateImage(5, VerificationType.Numeric);
            }
        }

        /// <summary>
        /// 验证码的类型
        /// </summary>
        public enum VerificationType
        {
            /// <summary>
            /// 只有数字
            /// </summary>
            Numeric,
            /// <summary>
            /// 数字和英文字符
            /// </summary>
            NumericAndChar,
            /// <summary>
            /// 中文字符
            /// </summary>
            ChineseChar
        }

        /// <summary>
        /// 生成一个随机文字图片,保存在 Session["code1"]
        /// </summary>
        /// <param name="count">图片中字的个数</param>
        /// <returns>生成的文字</returns>
        public string CreateImage(int count, VerificationType type)
        {
            string ValidCode = GenCode(count, type);

            switch (type)
            {
                case VerificationType.Numeric:
                    CreateCheckCodeImage(ValidCode, 13.5);
                    break;
                case VerificationType.NumericAndChar:
                    CreateCheckCodeImage(ValidCode, 14);
                    break;
                case VerificationType.ChineseChar:
                    CreateCheckCodeImage(ValidCode, 22.5);
                    break;
                default:
                    break;
            }
            return ValidCode;
        }
        /// <summary>
        /// 产生随机字符串
        /// </summary>
        /// <param name="num">随机出几个字符</param>
        /// <returns>随机出的字符串</returns>
        private string GenCode(int num, VerificationType type)
        {
            string str = string.Empty;
            switch (type)
            {
                case VerificationType.Numeric:
                    str = "0123456789";
                    break;
                case VerificationType.NumericAndChar:
                    str = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
                    break;
                case VerificationType.ChineseChar:
                    break;
                default:
                    str = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
                    break;
            }
            char[] chastr = str.ToCharArray();
            string code = "";
            Random rd = new Random();
            int i;
            for (i = 0; i < num; i++)
            {
                code += str.Substring(rd.Next(0, str.Length), 1);
            }
            return code;
        }

        /// <summary>
        /// 生成图片（增加背景噪音线、前景噪音点）
        /// </summary>
        /// <param name="checkCode">随机出字符串</param>
        private void CreateCheckCodeImage(string checkCode, double codeWidth)
        {
            if (checkCode.Trim() == "" || checkCode == null)
                return;
            System.Drawing.Bitmap image = new System.Drawing.Bitmap((int)(checkCode.Length * codeWidth), 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
                int i;
                for (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("Verdana", 12, (System.Drawing.FontStyle.Bold));
                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, new System.Drawing.StringFormat());
                //画图片的前景噪音点
                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.Jpeg);
                HttpContext.Current.Response.ExpiresAbsolute = System.DateTime.Now.AddMilliseconds(0);
                //清除该页输出缓存，设置该页无缓存
                Response.Buffer = true;
                Response.ExpiresAbsolute = System.DateTime.Now.AddMilliseconds(0);
                Response.Expires = 0;
                Response.CacheControl = "no-cache";
                Response.AppendHeader("Pragma", "No-Cache");
                HttpContext.Current.Response.ClearContent();
                HttpContext.Current.Response.ContentType = "image/JPEG";
                HttpContext.Current.Response.BinaryWrite(ms.ToArray());
                g.Dispose();
                image.Dispose();
            }
            catch
            {
                g.Dispose();
                image.Dispose();
            }
        }