date time extensions

DateTimeExtensions.cs

class DateTimeExtensions
    {
        /// <summary>
        /// Adds/subtruct business days to date
        /// pass negative number to subtruct
        /// </summary>
        /// <param name="date">date that we are adding/subtructing days to</param>
        /// <param name="days">number of days</param>
        /// <returns>found datetime</returns>
        public DateTime AddBusinessDays(DateTime date, int days)
        {

            if (days == 0) return date;

            if (date.DayOfWeek == DayOfWeek.Saturday)
            {
                date = date.AddDays(2);
                days -= 1;
            }
            else if (date.DayOfWeek == DayOfWeek.Sunday)
            {
                date = date.AddDays(1);
                days -= 1;
            }

            date = date.AddDays(days / 5 * 7);
            int extraDays = days % 5;

            if ((int)date.DayOfWeek + extraDays > 5)
            {
                extraDays += 2;
            }

            return date.AddDays(extraDays);
        }

        /// <summary>
        /// select all weekdays from odd/even/userInput weeks in a date range
        /// </summary>
        /// <param name="from">date range start</param>
        /// <param name="to">date range to</param>
        /// <param name="choice">odd, even, or userInput</param>
        /// <param name="dayOfWeeks">which days of week we want to choose</param>
        /// <param name="usetInputWeeks">user chooses usrInput, he must pass list of integers</param>
        /// <returns></returns>
        public List<DateTime> GetDates(DateTime from, DateTime to, string choice, List<int> dayOfWeeks, List<int> usetInputWeeks = null)
        {
            var dateTimeToReturn = new List<DateTime>();
            var calendar = new GregorianCalendar();
            var fromWeekNum = calendar.GetWeekOfYear(from, CalendarWeekRule.FirstDay, DayOfWeek.Monday);
            var toWeekNum = calendar.GetWeekOfYear(to, CalendarWeekRule.FirstDay, DayOfWeek.Monday);
            var weekNums = new List<int>();
            var range = Enumerable.Range(fromWeekNum, toWeekNum + 1).ToList();
            switch (choice)
            {
                case "odd":
                    weekNums = range.Where(x => x % 2 != 0).ToList();
                    break;
                case "even":
                    weekNums = range.Where(x => x % 2 == 0).ToList();
                    break;
                case "userInput":
                    if (usetInputWeeks != null)
                    {
                        weekNums = usetInputWeeks;
                    }
                    break;
                default:
                    break;
            }
                        
            while (from <= to)
            {
                var weekNum = calendar.GetWeekOfYear(from, CalendarWeekRule.FirstDay, DayOfWeek.Monday);

                if (weekNums.Contains(weekNum) && dayOfWeeks.Contains((int)from.DayOfWeek))
                {
                    dateTimeToReturn.Add(from);
                }
                from = from.AddDays(1);
            }       
            return dateTimeToReturn;
        }
    }