public void Library_AddAuthor_Correct() { // Arrage List <Author> authors = new List <Author>() { new Author() { Id = 1, Name = "Name0", Surname = "Surname0" }, new Author() { Id = 2, Name = "Name1", Surname = "Surname1" }, new Author() { Id = 3, Name = "Name2", Surname = "Surname2" }, }; Mock <IDataProvider> data = new Mock <IDataProvider>(); data.Setup(p => p.GetAuthors()).Returns(authors); ILibrary library = new LibraryCollection(data.Object); Author newAuthor = new Author() { Id = 20, Name = "Name10", Surname = "Surname10" }; // Act authors.Add(newAuthor); library.AddAuthor(newAuthor); // Assert Assert.Equal(authors, library.GetAuthors()); }