How to Get the number of weeks in a given year

I had the issue when I assign 2018 to year parameter of other methods, so I extended the Code of Tim Schmelter. I do not know, maybe there are codes that work faster:

//you can try like that 
int weeks = DateHelper.GetWeeksInGivenYear(2018);
int weeks = DateHelper.GetWeeksInGivenYear(2020);

// This presumes that weeks start with Monday.
// Week 1 is the 1st week of the year with a Thursday in it.
public static int GetIso8601WeekOfYear(this DateTime time)
{
    // Seriously cheat.  If its Monday, Tuesday or Wednesday, then it'll 
    // be the same week# as whatever Thursday, Friday or Saturday are,
    // and we always get those right
    DayOfWeek day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
    if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
    {
        time = time.AddDays(3);
    }

    // Return the week of our adjusted day
    return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}

//gets given year last week no
public static int GetWeeksInGivenYear(int year)
{
    DateTime lastDate = new DateTime(year, 12, 31);
    int lastWeek = GetIso8601WeekOfYear(lastDate);

    while (lastWeek == 1)
    {
        lastDate = lastDate.AddDays(-1);
        lastWeek = GetIso8601WeekOfYear(lastDate);

    }
    return lastWeek;
}

See the Calendar.GetWeekOfYear method

public int GetWeeksInYear(int year)
{
      DateTimeFormatInfo dfi = DateTimeFormatInfo.CurrentInfo;
      DateTime date1 = new DateTime(year, 12, 31);
      Calendar cal = dfi.Calendar;
      return  cal.GetWeekOfYear(date1, dfi.CalendarWeekRule, 
                                          dfi.FirstDayOfWeek);
}

Be carefull to figure out the correct CalendarWeekRule and FirstDayOfWeek for a Calendar that matches the culture your customers are used to. (for some calenders it might vary...)

Update 14 Oct 2019

If you're using .NET Core 3.0 and you want to get the number of weeks in a year conforming to ISO 8601 - you can use ISOWeek's GetWeeksInYear method.

using System;
using System.Globalization;

public class Program
{
    public static void Main()
    {
        Console.WriteLine(ISOWeek.GetWeeksInYear(2009)); // returns 53
    }
}

Working example: https://dotnetfiddle.net/EpIbZQ