public void slotNumbersForCarswithColor_NegTest() { Car[] cars = new Car[5] { new Car { Color = "white", RegistrationNumber = "KA-01-HH-1234", SlotNumber = 1 }, new Car { Color = "red", RegistrationNumber = "KA-01-HH-9876", SlotNumber = 2 }, new Car { Color = "blue", RegistrationNumber = "KA-01-KH-1762", SlotNumber = 3 }, null, new Car { Color = "blue", RegistrationNumber = "KA-01-ET-1111", SlotNumber = 5 } }; sampleLot = new Model.ParkingLot(4) { Cars = cars }; ICommands commands = new CommandRouter(); Program.ParkingLot = sampleLot; List <int> slotNos = commands.slot_numbers_for_cars_with_colour("purple"); Assert.IsTrue(slotNos == null); }