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

Based on the comment marked as answer and patch recommended , as well as -> This version wants to convert the Days to Business-Hours ... Considers Same day hours as well.

 /// 

    /// Calculates number of business days, taking into account:
    ///  - weekends (Saturdays and Sundays)
    ///  - bank holidays in the middle of the week
    /// 
    /// First day in the time interval
    /// Last day in the time interval
    /// List of bank holidays excluding weekends
    /// Number of business hours during the 'span'
    public static int BusinessHoursUntil(DateTime firstDay, DateTime lastDay, params DateTime[] bankHolidays)
    {
        var original_firstDay = firstDay;
        var original_lastDay = lastDay;
        firstDay = firstDay.Date;
        lastDay = lastDay.Date;
        if (firstDay > lastDay)
            return -1; //// throw new ArgumentException("Incorrect last day " + lastDay);

        TimeSpan span = lastDay - firstDay;
        int businessDays = span.Days + 1;
        int fullWeekCount = businessDays / 7;
        // find out if there are weekends during the time exceedng the full weeks
        if (businessDays > fullWeekCount * 7)
        {
            // we are here to find out if there is a 1-day or 2-days weekend
            // in the time interval remaining after subtracting the complete weeks
            int firstDayOfWeek = firstDay.DayOfWeek == DayOfWeek.Sunday ? 7 : (int)firstDay.DayOfWeek;
            int lastDayOfWeek = lastDay.DayOfWeek == DayOfWeek.Sunday ? 7 : (int)lastDay.DayOfWeek;

            if (lastDayOfWeek < firstDayOfWeek)
                lastDayOfWeek += 7;
            if (firstDayOfWeek <= 6)
            {
                if (lastDayOfWeek >= 7)// Both Saturday and Sunday are in the remaining time interval
                    businessDays -= 2;
                else if (lastDayOfWeek >= 6)// Only Saturday is in the remaining time interval
                    businessDays -= 1;
            }
            else if (firstDayOfWeek <= 7 && lastDayOfWeek >= 7)// Only Sunday is in the remaining time interval
                businessDays -= 1;
        }

        // subtract the weekends during the full weeks in the interval
        businessDays -= fullWeekCount + fullWeekCount;

        if (bankHolidays != null && bankHolidays.Any())
        {
            // subtract the number of bank holidays during the time interval
            foreach (DateTime bankHoliday in bankHolidays)
            {
                DateTime bh = bankHoliday.Date;
                if (firstDay <= bh && bh <= lastDay)
                    --businessDays;
            }
        }

        int total_business_hours = 0;
        if (firstDay.Date == lastDay.Date)
        {//If on the same day, go granular with Hours from the Orginial_*Day values
            total_business_hours = (int)(original_lastDay - original_firstDay).TotalHours;
        }
        else
        {//Convert Business-Days to TotalHours
            total_business_hours = (int)(firstDay.AddDays(businessDays).AddHours(firstDay.Hour) - firstDay).TotalHours;
        }
        return total_business_hours;
    }