public void ShouldReturnExistingBookFromRepositoryIfExists() { var existingBook = new Book { Id = 0, Title = "Gra Endera", Author = "Orson Scott Card", Localisation = "Gliwice", ISBN = "9788376482514" }; var booksInLibrary = new List <Book> { existingBook, new Book { Id = 1, Title = "Cieñ Endera", Author = "Orson Scott Card", Localisation = "Gliwice", ISBN = "9788378397649" } }; _booksDbSetMock.As <IQueryable <Book> >().Setup(x => x.ElementType).Returns(booksInLibrary.AsQueryable().ElementType); _booksDbSetMock.As <IQueryable <Book> >().Setup(x => x.Expression).Returns(booksInLibrary.AsQueryable().Expression); _booksDbSetMock.As <IQueryable <Book> >().Setup(x => x.Provider).Returns(booksInLibrary.AsQueryable().Provider); _booksDbSetMock.As <IQueryable <Book> >().Setup(x => x.GetEnumerator()).Returns(booksInLibrary.GetEnumerator()); var bookWithGivenIsbn = _libraryRepository.FindBookWithGivenIsbn("9788376482514"); Assert.Equal(existingBook, bookWithGivenIsbn); }