public void Library_AddBook_Correct() { // Arrange List <Book> books = new List <Book>() { new Book() { Id = 1, Name = "Book0" }, new Book() { Id = 2, Name = "Book1" }, }; Mock <IDataProvider> data = new Mock <IDataProvider>(); data.Setup(p => p.GetBooks()).Returns(books); ILibrary library = new LibraryCollection(data.Object); Book newBook = new Book() { Id = 12, Name = "Book12" }; // Act library.AddBook(newBook); books.Add(newBook); // Assert Assert.Equal(books, library.GetBooks()); }