生成日期流水号帮助类

 /// <summary>
    /// 生成日期流水号
    /// </summary>
    public class DateSerialNumberGenerator
    {
        private static readonly object Lock = new object();

        private static string _nowYear;//系统年份

        private static string _nowMonth;//系统月份

        /// <summary>
        /// 获取下一个日期流水号 , 步长=1
        /// </summary>
        /// <param name="thisSNumber">当前流水 - 位长[>=7] - 格式[YYYYMMXXX]</param>
        /// <param name="minSNumberLength">字串位长,默认=7</param>
        /// <returns></returns>
        public static string GetNextSNumber(string thisSNumber = null, int minSNumberLength = 9)
        {
            //获取系统时间
            _nowYear = DateTime.Now.Year.ToString();

            _nowMonth = DateTime.Now.Month.ToString().PadLeft(2, '0');

            //为空生成当年/月第一个序列号
            if (string.IsNullOrEmpty(thisSNumber) || thisSNumber.Length < minSNumberLength)
            {
                return _nowYear + _nowMonth + "1".PadLeft(minSNumberLength - _nowYear.Length - _nowMonth.Length, '0');
            }

            lock (Lock)
            {
                //截取参数时间
                var year = thisSNumber.Substring(0, 4);

                var month = thisSNumber.Substring(4, 2);

                var num = thisSNumber.Substring(6);

                //跨年判定
                if (year.Equals(_nowYear))
                {
                    //跨月判定
                    if (month.Equals(_nowMonth))
                    {
                        return year + month + GetFormatNum(num, thisSNumber.Length);
                    }
                    return year + _nowMonth + "1".PadLeft(thisSNumber.Length - _nowYear.Length - _nowMonth.Length, '0');
                }
                return _nowYear + _nowMonth + "1".PadLeft(thisSNumber.Length - _nowYear.Length - _nowMonth.Length, '0');
            }
        }

        /// <summary>
        /// 位移格式化序列号
        /// </summary>
        /// <param name="num">序列号</param>
        /// <param name="numberlenght">字串位长</param>
        /// <returns></returns>
        private static string GetFormatNum(string num, int numberlenght)
        {
            string nowNum = (Convert.ToInt32(num) + 1).ToString();

            return nowNum.PadLeft(numberlenght - _nowYear.Length - _nowMonth.Length, '0');
        }
    }

结果：

201807001