C# 得到本周的第一天和最后一天

1、得到本周的第一天和最后一天

 /// <summary>
         /// 得到本周第一天(以星期一为第一天)
         /// </summary>
         /// <param name="datetime"></param>
         /// <returns></returns>
         public DateTime GetWeekFirstDayMon(DateTime datetime)
         {
             //星期一为第一天
             int weeknow = Convert.ToInt32(datetime.DayOfWeek);

             //因为是以星期一为第一天，所以要判断weeknow等于0时，要向前推6天。
             weeknow = (weeknow == 0 ? (7 - 1) : (weeknow - 1));
             int daydiff = (-1) * weeknow;

             //本周第一天
             string FirstDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
             return Convert.ToDateTime(FirstDay);
         }
         /// <summary>
         /// 得到本周最后一天(以星期天为最后一天)
         /// </summary>
         /// <param name="datetime"></param>
         /// <returns></returns>
         public DateTime GetWeekLastDaySun(DateTime datetime)
         {
             //星期天为最后一天
             int weeknow = Convert.ToInt32(datetime.DayOfWeek);
             weeknow = (weeknow == 0 ? 7 : weeknow);
             int daydiff = (7 - weeknow);

             //本周最后一天
             string LastDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
             return Convert.ToDateTime(LastDay);
         }