public void AddReadertoRent() { // create a repository DataRepository dr = new DataRepository(); // create data Book book = new Book("Needful Things", "King, Stephen"); Reader reader = new Reader("xyz", "Ossowskiego 665", "1111111"); Reader reader1 = new Reader("zyx", "ulica01", "009"); Rent rent = new Rent(book, reader1, "03/06/2017"); // add reader to a rent Assert.IsTrue(rent.AddReader(reader)); // add data to the repository dr.CreateReader(reader); dr.CreateRent(rent); // expected value: true Assert.AreEqual("xyz", dr.Rents[0].reader.nameSurname); // expected NullReferenceException Assert.AreEqual("Needful Things", dr.Rents[0].rentedBook.title); }