protected override bool isValid(out string exceptionMessage) { if (distance <= 0) { return makeExceptionMessage (out exceptionMessage, "The distance between the locations is invalid"); } LocationRequest lr = new LocationRequest (); if (lr.fetchLocationFromAirports (start_airport, destination_airport).Count () != 0) { return makeExceptionMessage(out exceptionMessage, "There is already an traject between the given airports"); } return validAirport(start_airport, out exceptionMessage) && validAirport(destination_airport, out exceptionMessage); }