Calculate working days (per country)

DateTimeExtensions.cs

  public static DateTime AddWorkingDays(this DateTime date, int workingDaysToAdd, string countryCode = null)
  {
      return AddWorkingDays(date, workingDaysToAdd, (dateToTest) => IsWorkingDay(dateToTest, countryCode));
  }

  public static DateTime AddWorkingDays(this DateTime date, int workingDaysToAdd, Func<DateTime,bool> isWorkingDay)
  {
      var sign = Math.Sign(workingDaysToAdd);
      var unsignedDays = Math.Abs(workingDaysToAdd);
      for (var i = 0; i < unsignedDays; i++)
      {
          do
          {
              date = date.AddDays(sign);
          }
          while (!isWorkingDay(date));
      }
      return date;
  }
  
  public static bool IsWorkingDay(this DateTime date, string countryCode = null)
  {
      return !IsNonWorkDay(date.DayOfWeek, countryCode);
  }

  private static bool IsNonWorkDay(DayOfWeek day, string countryCode = null)
  {
      switch (countryCode)
      {
          case "DZ": // Algeria
          case "BH": // Bahrain
          case "BD": // Bangladesh
          case "EG": // Egypt
          case "IQ": // Iraq
          case "IL": // Israel
          case "JO": // Jordan
          case "KW": // Kuwait
          case "LY": // Libya
          case "MV": // Maldives
          case "MR": // Mauritania
          case "OM": // Oman
          case "QA": // Qatar
          case "SA": // Saudi Arabia
          case "SD": // Sudan
          case "SY": // Syria
          case "AE": // U.A.E.
          case "YE": // Yemen
              return day == DayOfWeek.Friday || day == DayOfWeek.Saturday;

          case "AF": // Afghanistan
              return day == DayOfWeek.Thursday || day == DayOfWeek.Friday;

          case "IR": // Iran
              return day == DayOfWeek.Friday;

          case "NP": // Nepal
              return day == DayOfWeek.Saturday;

          case "BN": // Brunei Darussalam
              return day == DayOfWeek.Friday || day == DayOfWeek.Sunday;

          case "IN": // India
          case "TH": // Thailand
              return day == DayOfWeek.Sunday;
      }
      // most common Saturday/Sunday
      return day == DayOfWeek.Saturday || day == DayOfWeek.Sunday;
  }