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);  
          }  
      }