传入一个日期，获取日期是当月的第几周

 　　没找到可以直接用的函数，只能自己写咯。

 private int getWeek(DateTime daytime)
        {
            int dayInMonth = daytime.Day;
            //本月第一天
            DateTime firstDay = daytime.AddDays(1 - daytime.Day);
            //本月第一天是周几
            int weekday = (int)firstDay.DayOfWeek == 0 ? 7 : (int)firstDay.DayOfWeek;
            //本月第一周有几天
            int firstWeekEndDay = 7 - (weekday - 1);
            //当前日期和第一周之差
            int diffday = dayInMonth - firstWeekEndDay;
            diffday = diffday > 0 ? diffday : 1;
            //当前是第几周,如果整除7就减一天
            int WeekNumInMonth = ((diffday % 7) == 0
             ? (diffday / 7 - 1)
             : (diffday / 7)) + 1 + (dayInMonth > firstWeekEndDay ? 1 : 0);
            return Week;
        }