比较全的验证码类（数字，字母，汉字）

  protected void Page_Load(object sender, EventArgs e)
        {
            string gif = string.Empty;
            switch (Request.QueryString["op"])
            {
                case "1":   //汉字
                    gif = stxt(10);
                    Session["gif"] = stxt(4);
                    break;
                case "2":   //数字
                    gif = GetRandomint(4);
                    Session["gif"] = GetRandomint(4);
                    break;
                case "3":   //混合
                    gif = CreateRandomCode(4);
                    Session["gif"] = CreateRandomCode(4);
                    break;
                default:   //默认
                    gif = CreateRandomCode(4);
                    Session["gif"] = CreateRandomCode(4);
                    break;
            }
            CreateImage(Session["gif"].ToString());
        }

        /// <summary>
        /// 产生一个随机数字
        /// </summary>
        /// <param name="codeCount">数字的位数</param>
        /// <returns>数字字符串</returns>
        private String GetRandomint(int codeCount)
        {
            Random random = new Random();
            string min = "";
            string max = "";
            for (int i = 0; i < codeCount; i++)
            {
                min += "1";
                max += "9";
            }
            return (random.Next(Convert.ToInt32(min), Convert.ToInt32(max)).ToString());
        }

        /// <summary>
        /// 产生一个 随机中文汉字编码 
        /// 
        /// 此函数在汉字编码范围内随机创建含两个元素的十六进制字节数组，每个字节数组代表一个汉字，并将 
        /// 四个字节数组存储在object数组中。 
        /// 
        /// </summary>
        /// <param name="strlength">需要产生的汉字个数 </param>
        /// <returns></returns>
        public static object[] CreateRegionCode(int strlength)
        {
            //定义一个字符串数组储存汉字编码的组成元素 
            string[] rBase = new String[16] { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "a", "b", "c", "d", "e", "f" };
            Random rnd = new Random();
            //定义一个object数组用来 
            object[] bytes = new object[strlength];

            /**/