生成0-9随机数

///

        /// 生成0-9随机数
        ///

        /// 生成长度
        ///
        public static string RndNum(int codeNum)
        {
            StringBuilder sb = new StringBuilder(codeNum);
            Random rand = new Random();
            for (int i = 1; i < codeNum + 1; i++)
            {
                int t = rand.Next(9);
                sb.AppendFormat("{0}", t);
            }
            return sb.ToString();

        }