c#自定义WeekOfYear函数

昨天用公式实现某一日期是第几周的计算，今天用c#实现。

        private int WeekOfYear(DateTime dt)
        {
            string firstdayofyear = dt.Year.ToString() + "-01-01";
            DateTime firstday = Convert.ToDateTime(firstdayofyear);
            int weekday=(int)firstday.DayOfWeek;
            int weeknum = (dt.DayOfYear + weekday - 2) / 7 + 1;
            return weeknum;
        }