gistfile1.cs

public class BusinessDayCalculator : IBusinessDayCalculator
{
private readonly IHolidayProvider _holidayProvider;

public BusinessDayCalculator(IHolidayProvider holidayProvider)
{
_holidayProvider = holidayProvider;
}

public DateTime AddBusinessDays(DateTime startDate, int businessDays)
{
var holidays = _holidayProvider.GetHolidays(startDate) ?? new List<Holiday>();

var listOfBusinessDays = new List<DateTime>();
var currentBusinessDayNumber = 1;
var currentDayNumber = 1;

while (currentBusinessDayNumber <= Math.Abs(businessDays))
{
var dateCandidate = startDate.AddDays(currentDayNumber * Math.Sign(businessDays));

var notWeekend = dateCandidate.DayOfWeek != DayOfWeek.Saturday
&& dateCandidate.DayOfWeek != DayOfWeek.Sunday;

var notHoliday = !holidays.Any(x => x.Date == dateCandidate);

if (notWeekend && notHoliday)
{
currentBusinessDayNumber++;
listOfBusinessDays.Add(dateCandidate);
}

currentDayNumber++;
}

return listOfBusinessDays.Last();
}
}

public interface IHolidayProvider
{
List<Holiday> GetHolidays(DateTime startDate);
}