获得随机文件名

/// <summary>
    /// 获得随机文件名
    /// </summary>
    /// <param name="filename"></param>
    /// <returns></returns>
    public static string GetRandomFileName(string filename)
    {
        string[] files = filename.Split('.');
        string exfilename = "." + files.GetValue(files.Length - 1);

        char[] s = new char[]{'0','1', '2','3','4','5','6','7','8','9','A'
          ,'B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q'
          ,'R','S','T','U','V','W','X','Y','Z'};
        string num = "";
        Random r = new Random();
        for (int i = 0; i < 4; i++)
            num += s[r.Next(0, s.Length)].ToString();
        DateTime time = DateTime.Now;
        string name = time.Year.ToString()
            + time.Month.ToString().PadLeft(2, '0')
            + time.Day.ToString().PadLeft(2, '0')
            + time.Hour.ToString().PadLeft(2, '0')
            + time.Minute.ToString().PadLeft(2, '0')
            + time.Second.ToString().PadLeft(2, '0')
            + num + exfilename;
        return name;
    }