关键代码实现<C#>

      if (Date != "")
            {
                switch (Date)
                {
                    case "0":// 查询今日的记录
                        string today = DateTime.Now.ToString("yyyyMMdd"); 
                        str += " And CSettlement_time='" + today + "' ";
                        break;
                    case "1":// 查询昨日的记录
                        today = DateTime.Now.ToString("yyyyMMdd");
                        string yestoday = DateTime.Now.AddDays(-1).ToString("yyyyMMdd"); 
                        str += " And CSettlement_time>='" + yestoday + "' And CSettlement_time<'"+ today + "'  ";
                        break;
                    case "2":// 查询近两日的记录
                        yestoday = DateTime.Now.AddDays(-1).ToString("yyyyMMdd");
                        str += " And CSettlement_time>='" + yestoday + "' ";
                        break;
                    case "3":// 查询本周的记录
                        Double day = Convert.ToDouble((0 - Convert.ToInt16(DateTime.Now.DayOfWeek)));
                        string week = DateTime.Now.AddDays(Convert.ToDouble((0 - Convert.ToInt16(DateTime.Now.DayOfWeek)))).ToString("yyyyMMdd");
                        str += " And CSettlement_time>='" + week + "' ";
                        break;
                    case "4":// 查询上周的记录
                        day = Convert.ToDouble((0 - Convert.ToInt16(DateTime.Now.DayOfWeek))) - 7;
                        week = DateTime.Now.AddDays(Convert.ToDouble((0 - Convert.ToInt16(DateTime.Now.DayOfWeek)))).ToString("yyyyMMdd");
                        string yesweek = DateTime.Now.AddDays(Convert.ToDouble((0 - Convert.ToInt16(DateTime.Now.DayOfWeek))) - 7).ToString("yyyyMMdd");
                        str += " And CSettlement_time>='" + yesweek + "' And CSettlement_time<'" + week + "' ";
                        break;
                    case "5":// 查询近一周的记录
                        week = DateTime.Now.AddDays(-7).ToString("yyyyMMdd");
                        str += " And CSettlement_time>='" + week + "' ";
                        break;
                    case "6":// 查询本月的记录
                        string month = DateTime.Now.ToString("yyyyMM01");
                        str += " And CSettlement_time>='" + month + "' ";
                        break;
                    case "7":// 查询上个月的记录
                        month = DateTime.Now.ToString("yyyyMM01");
                        string yesmonth = DateTime.Parse(DateTime.Now.ToString()).AddMonths(-1).ToString("yyyyMM01");
                        str += " And CSettlement_time>='" + yesmonth + "' And CSettlement_time<'" + month + "' ";
                        break;
                    case "8":// 查询近一个月的记录
                        month = DateTime.Parse(DateTime.Now.ToString()).AddMonths(-1).ToString("yyyyMMdd");
                        str += " And CSettlement_time>='" + month + "' ";
                        break;
                    case "9":// 查询近两个月的记录
                        month = DateTime.Parse(DateTime.Now.ToString()).AddMonths(-2).ToString("yyyyMMdd");
                        str += " And CSettlement_time>='" + month + "' ";
                        break;
                    case "10":// 查询近三个月的记录
                        month = DateTime.Parse(DateTime.Now.ToString()).AddMonths(-3).ToString("yyyyMMdd");
                        str += " And CSettlement_time>='" + month + "' ";
                        break;
                }
            }

作者：黑乌鸦-Brook-阿木

出处：https://www.jianshu.com/p/f223bc64699c
参考：
C#获取前一天日期

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