using System;

public class Program

{

    public static void Main()

    {

        var result = GetEndDateTransaction(new DateTime(2017,12,15), 8, 7);

        Console.WriteLine("Result: " + result);

        //Console.WriteLine("------------------------");

    }

    public static DateTime GetEndDateTransaction(DateTime startDate, int duration, int scheduleType)

    {

        DateTime result = GetDefaultEndDate(startDate, duration);

​

        switch ((int)scheduleType)

        {

            case 4:

            result = GetEndtDateForShift4(startDate, duration);

            break;

            case 5:

            result = GetEndtDateForShift5(startDate, duration);

            break;

            case 6:

            result = GetEndtDateForShift6(startDate, duration);

            break;

        }

        return result.Date;

    }

    private static DateTime GetDefaultEndDate(DateTime start, int duration)

    {

        DateTime newDate = start.AddDays(-1);

        newDate = newDate.AddDays(duration);

        return newDate;

    }

    private static DateTime GetEndtDateForShift4(DateTime start, int duration)

    {

        int direction = duration < 0 ? -1 : 1;

        DateTime newDate = start.AddDays(-1);

        while (duration != 0)

        {

            newDate = newDate.AddDays(direction);

            if (newDate.DayOfWeek != DayOfWeek.Friday &&

                newDate.DayOfWeek != DayOfWeek.Saturday &&

                newDate.DayOfWeek != DayOfWeek.Sunday)

            {

                duration -= direction;

            }

        }

        return newDate;

    }

    private static DateTime GetEndtDateForShift5(DateTime start, int duration)

    {

        int direction = duration < 0 ? -1 : 1;

        DateTime newDate = start.AddDays(-1);

        while (duration != 0)

        {

            newDate = newDate.AddDays(direction);

            if (newDate.DayOfWeek != DayOfWeek.Saturday &&

                newDate.DayOfWeek != DayOfWeek.Sunday)

            {

                duration -= direction;

            }

        }

        return newDate;

    }

    private static DateTime GetEndtDateForShift6(DateTime start, int duration)

    {

        int direction = duration < 0 ? -1 : 1;

        DateTime newDate = start.AddDays(-1);

        while (duration != 0)

        {

            newDate = newDate.AddDays(direction);

            if (newDate.DayOfWeek != DayOfWeek.Sunday)

                duration -= direction;

        }

        return newDate;

    }

}