ISO Week Number by Date, Date by ISO Week Number

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

namespace ValidationLog
{
    class DateUtils
    {

        public static DateTime FirstDateOfWeek(string week_format_1)
        {
            Int32 week_key = 0;
            if (week_format_1.Trim().Length != 6 || !Int32.TryParse(week_format_1, out week_key))
            {
                throw new ArgumentException(String.Format("'{0}' is invalid week format",week_format_1));
            }

            int year = week_key / 100;
            int weekOfYear = week_key % 100;

            return FirstDateOfWeek(year,weekOfYear);
        }

        public static string WeekId(DateTime date)
        {
            int wk = 0;
            int year = 0;

            DateUtils.GetWeekNumber(date, out year, out wk);

            return String.Format("{0}_{1:00}", year, wk);
        }

        /// <summary>
        /// Returns date of first day of week
        /// </summary>
        /// <param name="year">week year</param>
        /// <param name="weekOfYear">week number</param>
        /// <seealso cref="http://stackoverflow.com/questions/662379/calculate-date-from-week-number"/>
        /// <returns>date</returns>
        public static DateTime FirstDateOfWeek(int year, int weekOfYear)
        {
            DateTime jan1 = new DateTime(year, 1, 1);

            int daysOffset = (int)CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek - (int)jan1.DayOfWeek;

            DateTime firstMonday = jan1.AddDays(daysOffset);

            //get first week by ISO
            int firstWeek = CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(jan1, CultureInfo.InvariantCulture.DateTimeFormat.CalendarWeekRule, CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek);

            if (firstWeek <= 1)
            {
                weekOfYear -= 1;
            }

            return firstMonday.AddDays(weekOfYear * 7);
        }

        /// <summary>
        /// Calculates ISO week and year number (ISO year can differ from calendar year)
        /// </summary>
        /// <param name="date">date</param>
        /// <param name="year">ISO year</param>
        /// <param name="wk">ISO week</param>
        /// <seealso cref="http://codebetter.com/petervanooijen/2005/09/26/iso-weeknumbers-of-a-date-a-c-implementation/"/>
        public static void GetWeekNumber(DateTime date, out int year, out int wk)
        {
            year = date.Year;
            // Get jan 1st of the year
            DateTime startOfYear = new DateTime(year, 1, 1);
            // Get dec 31st of the year
            DateTime endOfYear = new DateTime(year, 12, 31);
            // ISO 8601 weeks start with Monday 
            // The first week of a year includes the first Thursday 
            // DayOfWeek returns 0 for sunday up to 6 for saterday
            int[] iso8601Correction = { 6, 7, 8, 9, 10, 4, 5 };
            int nds = date.Subtract(startOfYear).Days + iso8601Correction[(int)startOfYear.DayOfWeek];
            wk = nds / 7;
            switch (wk)
            {
                case 0:
                    // Return weeknumber of dec 31st of the previous year
                    GetWeekNumber(startOfYear.AddDays(-1), out year, out wk);
                    break;
                case 53:
                    // If dec 31st falls before thursday it is week 01 of next year
                    if (endOfYear.DayOfWeek < DayOfWeek.Thursday)
                    {
                        wk = 1; year += 1;
                    }
                    break;
                default:
                    break;
            }
        }



        /*
        public static DateTime FirstDateOfWeekCustom(int year, int weekOfYear)
        {
            return new DateTime(1999, 12, 31);
        }
        */
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using ValidationLog;


namespace ConsoleApplication2010Lab
{
    class Program
    {
        static void Main(string[] args)
        {
            DateTime endDate = new DateTime(2013,1,8);
            DateTime startDate = new DateTime(2010,12,27);

            for (DateTime date = startDate; date < endDate; date = date.AddDays(7))
            {
                Console.WriteLine("{0} - {1}", DateUtils.WeekId(date), date);
            }

            Console.ReadLine();
        }
    }
}