Getting the last friday of the month as an extension method

gistfile1.cs

    public static class DateTimeExtensions
    {
        public static DateTime LastFridayOfMonth(this DateTime value)
        {
            var days = Enumerable.Range(1, DateTime.DaysInMonth(value.Year, value.Month));

            var dates = days.Select(day => new DateTime(value.Year, value.Month, day));

            var lastFriday = dates.OrderBy(day => day).Last(day => day.DayOfWeek == DayOfWeek.Friday);

            return lastFriday;
        }
    }

public static class DateTimeExtensions
{
    public static DateTime LastOfMonth(this DateTime value, DayOfWeek dayOfWeek)
    {
        var days = Enumerable.Range(1, DateTime.DaysInMonth(value.Year, value.Month));

        var dates = days.Select(day => new DateTime(value.Year, value.Month, day));

        var lastOfMonth = dates.Last(day => day.DayOfWeek == dayOfWeek);

        return lastOfMonth;
    }
}

Great idea.

lastFriday may be somewhat misleading after the refactoring.

I would probably write it like this:

public static class DateTimeExtensions
{
    public static DateTime LastOfMonth(this DateTime value, DayOfWeek dayOfWeek)
    {
        var days = Enumerable.Range(1, DateTime.DaysInMonth(value.Year, value.Month))
            .Select(day => new DateTime(value.Year, value.Month, day))
            .Last(day => day.DayOfWeek == dayOfWeek);
    }
}

Nice catch Troels, updated. I prefer to leave statements with ret vals. Easier to debug in the future if the need arises.

An alternative approach would be:

public static class DateTimeExtensions
{
    public static DateTime LastOfMonth(this DateTime value, DayOfWeek dayOfWeek)
    {
        var days = Enumerable.Range(1, DateTime.DaysInMonth(value.Year, value.Month))
            .Select(day => new DateTime(value.Year, value.Month, day))
            .Where(day => day.DayOfWeek == dayOfWeek)
            .Max();
    }
}