计算每月的第几周的周几

private DateTime GetWeekDay(DateTime dt, int weekIndex, int dayOfWeek)
        {
            DateTime tdt = new DateTime(dt.Year, dt.Month, 1);
            int firstDayOfWeekOfMonth = (int)(tdt.DayOfWeek);

            int addDay;
            int calWeekIndex = weekIndex;
            if (dayOfWeek < firstDayOfWeekOfMonth)
            {
                calWeekIndex++;
            }

            addDay = (7 - firstDayOfWeekOfMonth) + dayOfWeek + (calWeekIndex - 1) * 7;

            return tdt.AddDays(addDay);
        }