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

I followed User:Mark Ransom's algorithm and wrote a generalized day finder. For example to get the 3rd friday of december 2013,

int thirdFriday = DayFinder.FindDay(2013, 12, DayOfWeek.Friday, 3);

And here is the function definition. It doesn't have any iterative loops, so its efficient.

  public class DayFinder
  {

    //For example to find the day for 2nd Friday, February, 2016
    //=>call FindDay(2016, 2, DayOfWeek.Friday, 2)
    public static int FindDay(int year, int month, DayOfWeek Day, int occurance)
    {

        if (occurance <= 0 || occurance > 5)
            throw new Exception("Occurance is invalid");

        DateTime firstDayOfMonth = new DateTime(year, month, 1);
        //Substract first day of the month with the required day of the week 
        var daysneeded = (int)Day - (int)firstDayOfMonth.DayOfWeek;
        //if it is less than zero we need to get the next week day (add 7 days)
        if (daysneeded < 0) daysneeded = daysneeded + 7;
        //DayOfWeek is zero index based; multiply by the Occurance to get the day
        var resultedDay = (daysneeded + 1) + (7 * (occurance - 1));

        if (resultedDay > (firstDayOfMonth.AddMonths(1) - firstDayOfMonth).Days)
            throw new Exception(String.Format("No {0} occurance(s) of {1} in the required month", occurance, Day.ToString()));

        return resultedDay;
    }
}