C#获取周的第一天、最后一天、月第一天和最后一天

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    public class DateTimeTool
      {
          /// <summary>
          /// 获取指定日期所在周的第一天，星期天为第一天
          /// </summary>
          /// <param name="dateTime"></param>
          /// <returns></returns>
          public static DateTime GetDateTimeWeekFirstDaySun(DateTime dateTime)
          {
              DateTime firstWeekDay = DateTime.Now;  

              try
              {
                  //得到是星期几，然后从当前日期减去相应天数
                  int weeknow = Convert.ToInt32(dateTime.DayOfWeek);  

                  int daydiff = (-1) * weeknow;  

                  firstWeekDay = dateTime.AddDays(daydiff);
              }
              catch { }  

              return firstWeekDay;
          }  

          /// <summary>
          /// 获取指定日期所在周的第一天，星期一为第一天
          /// </summary>
          /// <param name="dateTime"></param>
          /// <returns></returns>
          public static DateTime GetDateTimeWeekFirstDayMon(DateTime dateTime)
          {
              DateTime firstWeekDay = DateTime.Now;  

              try
              {
                  int weeknow = Convert.ToInt32(dateTime.DayOfWeek);  

                  //星期一为第一天，weeknow等于0时，要向前推6天。
                  weeknow = (weeknow == 0 ? (7 - 1) : (weeknow - 1));  

                  int daydiff = (-1) * weeknow;  

                  firstWeekDay = dateTime.AddDays(daydiff);
              }
              catch { }  

              return firstWeekDay;
          }  

          /// <summary>
          /// 获取指定日期所在周的最后一天，星期六为最后一天
          /// </summary>
          /// <param name="dateTime"></param>
          /// <returns></returns>
          public static DateTime GetDateTimeWeekLastDaySat(DateTime dateTime)
          {
              DateTime lastWeekDay = DateTime.Now;  

              try
              {
                  int weeknow = Convert.ToInt32(dateTime.DayOfWeek);  

                  int daydiff = (7 - weeknow) - 1;  

                  lastWeekDay = dateTime.AddDays(daydiff);  

              }catch { }  

              return lastWeekDay;
          }  

          /// <summary>
          /// 获取指定日期所在周的最后一天，星期天为最后一天
          /// </summary>
          /// <param name="dateTime"></param>
          /// <returns></returns>  

          public static DateTime GetDateTimeWeekLastDaySun(DateTime dateTime)
          {
              DateTime lastWeekDay = DateTime.Now;  

              try
              {
                  int weeknow = Convert.ToInt32(dateTime.DayOfWeek);  

                  weeknow = (weeknow == 0 ? 7 : weeknow);  

                  int daydiff = (7 - weeknow);  

                  lastWeekDay = dateTime.AddDays(daydiff);
              }
              catch { }  

              return lastWeekDay;
          }  

          /// <summary>
          /// 获取指定日期的月份第一天
          /// </summary>
          /// <param name="dateTime"></param>
          /// <returns></returns>
          public static DateTime GetDateTimeMonthFirstDay(DateTime dateTime)
          {
              if (dateTime == null)
              {
                  dateTime = DateTime.Now;
              }  

              return new DateTime(dateTime.Year, dateTime.Month, 1);
          }  

          /// <summary>
          /// 获取指定月份最后一天
          /// </summary>
          /// <param name="dateTime"></param>
          /// <returns></returns>
          public static DateTime GetDateTimeMonthLastDay(DateTime dateTime)
          {
              int day = DateTime.DaysInMonth(dateTime.Year, dateTime.Month);  

              return new DateTime(dateTime.Year, dateTime.Month, day);
          }
      }