How do I get the month number from the year and week number in c#?

Question

As the title says, given the year and the week number, how do I get the month number?

edit: if a week crosses two months, I want the month the first day of the week

Answer 1

// Calculate the week number according to ISO 8601

    public static int Iso8601WeekNumber(DateTime dt)
    {
            return  CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(dt, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
    }

// ...

DateTime dt = DateTime.Now;

// Calculate the WeekOfMonth according to ISO 8601
int weekOfMonth = Iso8601WeekNumber(dt) - Iso8601WeekNumber(dt.AddDays(1 - dt.Day)) + 1;