生成0-9隨機數

/// <summary>
        /// 生成0-9隨機數
        /// </summary>
        /// <param name="codeNum">生成長度</param>
        /// <returns></returns>
        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();

        }