C# 獲取當前年份的週期，週期所在日期範圍

最近有一個項目要用到年份週期，用於數據統計圖表展現使用，當中用到年份週期,以及年份週期所在的日期範圍。當初設想經過已知數據來換算年份週期，通過搜索資料發現經過數據庫SQL語句來作，反而更加複雜。如今改變思路經過C#後臺代碼來算出兩段日期範圍中年份週期，在依據年份週期所對應的日期範圍進行數據庫查詢進行統計。須要解決如下兩個點問題，

第一點：依據日期查找所在年份的第幾周；

第二點：依據年份所在的週期計算出週期所在的日期範圍。

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication6
{
    class Program
    {
        static void Main(string[] args)
        {
            GregorianCalendar gc = new GregorianCalendar();
            int weekOfYear = gc.GetWeekOfYear(DateTime.Now, CalendarWeekRule.FirstDay, DayOfWeek.Monday);
            Console.WriteLine("當前第{0}周", weekOfYear);
            DateTime startDate, lastDate;
            for (int i = 1; i <= 53; i++)
            {

                GetDaysOfWeeks(DateTime.Now.Year, i, out startDate, out lastDate);
                Console.WriteLine("第{0}周", i);
                Console.WriteLine(startDate);
                Console.WriteLine(lastDate);
            }
            Console.ReadLine();
        }

        public static bool GetDaysOfWeeks(int year, int index, out DateTime first, out DateTime last)
        {
            first = DateTime.MinValue;
            last = DateTime.MinValue;
            if (year < 1700 || year > 9999)
            {
                //"年份超限"
                return false;
            }
            if (index < 1 || index > 53)
            {
                //"週數錯誤"
                return false;
            }
            DateTime startDay = new DateTime(year, 1, 1);  //該年第一天
            DateTime endDay = new DateTime(year + 1, 1, 1).AddMilliseconds(-1);
            int dayOfWeek = 0;
            if (Convert.ToInt32(startDay.DayOfWeek.ToString("d")) > 0)
                dayOfWeek = Convert.ToInt32(startDay.DayOfWeek.ToString("d"));  //該年第一天爲星期幾
            if (dayOfWeek == 0) { dayOfWeek = 7; }
            if (index == 1)
            {
                first = startDay.AddDays(7 - dayOfWeek - 6);
                if (dayOfWeek == 6)
                {
                    last = first;
                }
                else
                {
                    last = startDay.AddDays((7 - dayOfWeek));
                }
            }
            else
            {
                first = startDay.AddDays((8 - dayOfWeek) + (index - 2) * 7); //index周的起始日期
                last = first.AddDays(6);
                //if (last > endDay)
                //{
                //    last = endDay;
                //}
            }
            if (first > endDay)  //startDayOfWeeks不在該年範圍內
            {
                //"輸入週數大於本年最大週數";
                return false;
            }
            return true;
        }
    }
}

　　執行結果