C#获取日期是月中第几周

private  int  getWeekNumInMonth(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  WeekNumInMonth;
}