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

public static int CalculateBusinessDaysInRange(this DateTime startDate, DateTime endDate, params DateTime[] holidayDates)
{
    endDate = endDate.Date;
    if(startDate > endDate)
        throw new ArgumentException("The end date can not be before the start date!", nameof(endDate));
    int accumulator = 0;
    DateTime itterator = startDate.Date;
    do 
    {
        if(itterator.DayOfWeek != DayOfWeek.Saturday && itterator.DayOfWeek != DayOfWeek.Sunday && !holidayDates.Any(hol => hol.Date == itterator))
        { accumulator++; }
    } 
    while((itterator = itterator.AddDays(1)).Date <= endDate);
    return accumulator
}

I'm only posting this because despite all of the excellent answers that have been given, none of the math made sense to me. This is definitely a KISS method that should work and be fairly maintainable. Granted if you are calculating ranges that are greater than 2-3 months this will not be the most effective way. We simply determine if it is a Saturday or Sunday or the date is a given holiday date. If it's not we add a business day. If it is then everything is fine.

I'm sure this could be even more so simplified with LINQ, but this way is much easier to understand.