編程樂趣：C#擷取日期所在周、月份第一和最後一天

寫了個小功能，需要用到以周為時間段，於是寫了個擷取周第一和最後一天的方法，擷取月份的第一和最後一天就比較簡單了。代碼如下：

  public class DateTimeTool    {        /// <summary>        /// 擷取指定日期所在周的第一天，星期天為第一天        /// </summary>        /// <param name="dateTime"></param>        /// <returns></returns>        public static DateTime GetDateTimeWeekFirstDaySun(DateTime dateTime)        {            DateTime firstWeekDay = DateTime.Now;            try            {                //得到是星期幾，然後從當前日期減去相應天數                 int weeknow = Convert.ToInt32(dateTime.DayOfWeek);                int daydiff = (-1) * weeknow;                firstWeekDay = dateTime.AddDays(daydiff);            }            catch { }            return firstWeekDay;        }        /// <summary>        /// 擷取指定日期所在周的第一天，星期一為第一天        /// </summary>        /// <param name="dateTime"></param>        /// <returns></returns>        public static DateTime GetDateTimeWeekFirstDayMon(DateTime dateTime)        {            DateTime firstWeekDay = DateTime.Now;            try            {                int weeknow = Convert.ToInt32(dateTime.DayOfWeek);                //星期一為第一天，weeknow等於0時，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>        /// 擷取指定日期所在周的最後一天，星期六為最後一天        /// </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>        /// 擷取指定日期所在周的最後一天，星期天為最後一天        /// </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>        /// 擷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>        /// 擷取指定月份最後一天        /// </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);        }    }