public void GetRentalByRentalIdReturnsRightRental() { //Arrange IRentalRepository rentalRepository = Substitute.For <IRentalRepository>(); rentalRepository.GetRentalByRentalId(2).Returns(new Rental { CustomerName = "Lars" }); IUnitOfWork uow = Substitute.For <IUnitOfWork>(); uow.RentalRepository.Returns(rentalRepository); RentalService rs = new RentalService(uow); //Act var rental = rs.GetRentalByRentalId(2); //Assert rental.CustomerName.Should().Be("Lars"); }