How to find the 3rd Friday in a month with C#?

Question

Given a date (of type DateTime), how do I find the 3rd Friday in the month of that date?

Answer 1

This is a version that uses LINQ and functional programming style.

It works like this.

First, take all of the days of the month. Then select only the ones of the right day (Friday). Finally take the nth (3rd) entry and return.

// dt: The date to start from (usually DateTime.Now)
// n: The nth occurance (3rd)
// weekday: the day of the week to look for
    public DateTime GetNthWeekdayOfMonth(DateTime dt, int n, DayOfWeek weekday)
    {
        var days = Enumerable.Range(1, DateTime.DaysInMonth(dt.Year, dt.Month)).Select(day => new DateTime(dt.Year, dt.Month, day));

        var weekdays = from day in days
                            where day.DayOfWeek == weekday
                            orderby day.Day ascending
                            select day;

        int index = n - 1;

        if (index >= 0 && index < weekdays.Count())
            return weekdays.ElementAt(index);

        else
            throw new InvalidOperationException("The specified day does not exist in this month!");
   }