public void ReturnCorrectImagesCount_WhenCorrectBookIdIsPassed() { // Arrange var dbSetMock = new Mock <ILibrarySystemEfWrapper <Image> >(); var imageServices = new ImageServices(dbSetMock.Object); Guid id = Guid.NewGuid(); var expectedResult = 3; dbSetMock.Setup(r => r.All()).Returns(() => new List <Image>() { new Image() { Id = Guid.NewGuid(), BookId = id }, new Image() { Id = Guid.NewGuid(), BookId = id }, new Image() { Id = Guid.NewGuid(), BookId = id }, new Image() { Id = Guid.NewGuid(), BookId = Guid.NewGuid() } }.AsQueryable()); // Act var result = imageServices.GetImagesByBookId(id); // Assert Assert.AreEqual(expectedResult, result.ToList().Count()); }