public void TestGetAgeDifferenceError() { var personList = new List <Person> { new Person { Name = "George", Gender = "Male", DateOfBirth = Convert.ToDateTime("01/01/2001") }, new Person { Name = "Beck", Gender = "Male", DateOfBirth = Convert.ToDateTime("01/01/2000") } }; _addressBookLoader.Setup(c => c.LoadAddressBook()).Returns(personList); classUnderTest = new AddressBookService(_addressBookLoader.Object, _logger.Object); var result = classUnderTest.GetAgeDifference("Beck", "Jack"); Assert.Null(result.Result); }