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    /// </summary>
    /// <param name="n">生成长度为 n </param>
    /// <returns></returns>
    public static string RandNum()
    {
        char[] arrChar = new char[] { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9' };
        StringBuilder num = new StringBuilder();

        Random rnd = new Random(DateTime.Now.Millisecond);

        for (int i = 0; i < n; i++)
        {
            num.Append(arrChar[rnd.Next(0, 9)].ToString());

        }

        return num.ToString();
    }