Calculate the number of business days between two dates?

Question

In C#, how can I calculate the number of business (or weekdays) days between two dates?

Answer 1

I searched a lot for a, easy to digest, algorithm to calculate the working days between 2 dates, and also to exclude the national holidays, and finally I decide to go with this approach:

public static int NumberOfWorkingDaysBetween2Dates(DateTime start,DateTime due,IEnumerable holidays)
        {
            var dic = new Dictionary();
            var totalDays = (due - start).Days;
            for (int i = 0; i < totalDays + 1; i++)
            {
                if (!holidays.Any(x => x == start.AddDays(i)))
                    dic.Add(start.AddDays(i), start.AddDays(i).DayOfWeek);
            }

            return dic.Where(x => x.Value != DayOfWeek.Saturday && x.Value != DayOfWeek.Sunday).Count();
        } 

Basically I wanted to go with each date and evaluate my conditions:

1. Is not Saturday
2. Is not Sunday
3. Is not national holiday

but also I wanted to avoid iterating dates.

By running and measuring the time need it to evaluate 1 full year, I go the following result:

static void Main(string[] args)
        {
            var start = new DateTime(2017, 1, 1);
            var due = new DateTime(2017, 12, 31);

            var sw = Stopwatch.StartNew();
            var days = NumberOfWorkingDaysBetween2Dates(start, due,NationalHolidays());
            sw.Stop();

            Console.WriteLine($"Total working days = {days} --- time: {sw.Elapsed}");
            Console.ReadLine();

            // result is:
           // Total working days = 249-- - time: 00:00:00.0269087
        }

Edit: a new method more simple:

public static int ToBusinessWorkingDays(this DateTime start, DateTime due, DateTime[] holidays)
        {
            return Enumerable.Range(0, (due - start).Days)
                            .Select(a => start.AddDays(a))
                            .Where(a => a.DayOfWeek != DayOfWeek.Sunday)
                            .Where(a => a.DayOfWeek != DayOfWeek.Saturday)
                            .Count(a => !holidays.Any(x => x == a));

        }