获取当前周的周五的日期

 DateTime GetFridayDate(DateTime standardDate)
        {
            int weekNum = (int)standardDate.DayOfWeek;
            DateTime fridayDate = standardDate;

            if (weekNum > 0 && weekNum <= 5)
            {
                fridayDate = standardDate.AddDays(5 - weekNum);
            }
            else
            {
                if (weekNum == 6)
                {
                    fridayDate = standardDate.AddDays(-1);
                }
                else if (weekNum == 0)
                {
                    fridayDate = standardDate.AddDays(-2);
                }
            }

            return fridayDate;
        }