根据年月以及月中周次，获取该周开始，结束日期

注意，是该月中的周次，一周最多有6周。

        <span style="background-color: rgb(102, 51, 255);">//获取周开始日期和结束日期（周日-周六）
        public string GetWeekDate(int yearly, int monthly, string weekly)
        {
            string dayStr="";

            try
            {
                DateTime day1 = DateTime.Parse(yearly.ToString() + "-" + monthly.ToString() + "-01");//该月第一天日期
                int weekOfDay1 = (Int32)day1.DayOfWeek;//该月第一天是周几
                int dayCount = DateTime.DaysInMonth(yearly, monthly);//该月天数

                int endDay = 0;//结束日
                DateTime dateEnd = DateTime.Now;//周结束日期,初始化为当前时间

                //7*周次 - 第一周中上月天数 > 本月天数
                if (7 * Int32.Parse(weekly) - weekOfDay1 > dayCount)
                {
                    endDay = 7 * Int32.Parse(weekly) - weekOfDay1 - dayCount;
                    DateTime dateLast = DateTime.Parse(yearly.ToString() + "-" + monthly.ToString() + "-" + dayCount.ToString());//该月最后一天日期
                    dateEnd = dateLast.AddDays(endDay);
                }
                //7*周次 - 第一周中上月天数 > 本月天数
                if (7 * Int32.Parse(weekly) - weekOfDay1 <= dayCount)
                {
                    endDay = 7 * Int32.Parse(weekly) - weekOfDay1;
                    dateEnd = DateTime.Parse(yearly.ToString() + "-" + monthly.ToString() + "-" + endDay.ToString());
                }

                DateTime dateStart = dateEnd.AddDays(-6); //周开始日期

                dayStr = dateStart.ToString("yyyy-MM-dd") +" ~ " + dateEnd.ToString("yyyy-MM-dd");
            }
            catch (Exception)
            {
                throw;
            }
            return dayStr;
        }</span>

调用：var resultWeekDay = GetWeekDate(2015, 4, 5);//4月第5周

结果：2015-04-26  ~ 2015-05-02