编程乐趣：C#获取日期所在周、月份第一和最后一天

写了个小功能，需要用到以周为时间段，于是写了个获取周第一和最后一天的方法，获取月份的第一和最后一天就比较简单了。代码如下：

  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);
        }
    }