DNN Development Tips:6 - Dummies, Fakes, Stubs and Mocks
One of the tenets of Unit Testing is that in order to test a piece of code we often need to create some alternate implementation of one or more of the Interfaces involved in a test. This concept is generally called a Test Double. However there are many types of Test Double, and there is some degree of confusion about the types of doubles used.
In this DNN Development Tip I will attempt to clarify the types of Test Doubles.
Dummies
Dummies are the simplest type of test double – these are objects that are needed to be passed around but they don’t actually do anything so they can have a very simple implementation. Usually they are required just to fill a parameter list in a method call.
An example would be a test for the following constructor -
Figure 1: A Method that can be tested using a Dummy Test Double |
1: public CreateVocabularyPresenter(ICreateVocabularyView createView, 2: IVocabularyController vocabularyController, 3: IScopeTypeController scopeTypeController) 4: : base(createView) 5: { 6: Requires.NotNull("vocabularyController", vocabularyController); 7: Requires.NotNull("scopeTypeController", scopeTypeController); 8: 9: _vocabularyController = vocabularyController; 10: _scopeTypeController = scopeTypeController; 11: 12: View.Save += Save; 13: View.Model.Vocabulary = GetVocabulary(); 14: } |
In this example the constructor takes three parameters. If we are testing that the constructor throws an Exception if one of the arguments are null, we need to ensure that the other parameters are not null. So for example our test to ensure that the Constructor throws an Exception if the VocabularyController is null might look like this.
Figure 2: A Test using a Dummy Test Double |
1: public void Constructor_Requires_Non_Null_VocabularyController() 2: { 3: //Arrange 4: DummyCreateVocabularyView view = new DummyCreateVocabularyView(); 5: DummyScopeTypeController scopeTypeController = new DummyScopeTypeController(); 6: 7: //Act 8: CreateVocabularyPresenter presenter = new CreateVocabularyPresenter(view, 9: null, 10: scopeTypeController); 11: 12: //..... 13: } |
We don’t care about the implementation of the first (ICreateVocabularyView) or third (IScopeTypeController) interfaces in this test, we just need them to be non-null – all we care about is that the constructor throws if the second parameter is null.
Dummies are rarely used as we are often testing other behaviours in the same Test Fixture, so we might as well use one of the other types.
Fakes
The other three types of Test Doubles all have some form of implementation as we are testing some part of the Interface in question.
Fake objects usually have full working implementations, but they use some shortcut, and are not of production quality. An example of a Fake object would be when testing the business layer of an application, where we create an in-memory Fake DataService class to simulate the data access, and allow us to test the business layer methods, without requiring a database to be set up.
Stubs
Stubs provide canned answers to calls made during a test, and usually don’t respond at all to anything outside what's programmed in for the test. Stubs may also record information about calls, such as an email gateway stub that remembers the messages it 'sent', or maybe only how many messages it 'sent'.
Mocks
Mocks are usually dynamically created by a mocking framework – (in DNN we use Moq as the mocking framework). Mock objects are pre-programmed with expectations which form a specification of the calls they are expected to receive (see Figure 3)
Figure 3; An Example of using a Mock Object |
1: public void VocabularyListPresenter_On_Add_Redirects_To_CreateVocabulary() 2: { 3: // Arrange 4: Mock 5: Mock 6: view.Setup(v => v.Model).Returns(new VocabularyListModel()); 7: 8: Mock 9: Mock 10: httpContext.Setup(h => h.Response).Returns(httpResponse.Object); 11: 12: VocabularyListPresenter presenter = new VocabularyListPresenter(view.Object, mockController.Object) 13: { 14: HttpContext = httpContext.Object, 15: ModuleId = Constants.MODULE_ValidId, 16: TabId = Constants.TAB_ValidId 17: }; 18: 19: // Act (Raise the AddVocabulary Event) 20: view.Raise(v => v.AddVocabulary += null, EventArgs.Empty); 21: 22: // Assert 23: httpResponse.Verify(r => r.Redirect(Globals.NavigateURL(Constants.TAB_ValidId, 24: "CreateVocabulary", 25: String.Format("mid={0}", Constants.MODULE_ValidId)))); 26: } |
In Figure 3 we are using 4 separate Mock objects, but lets focus on the two mocks created in lines 8 and 9. In line 8 we create a Mock HttpContextBase using Moq’s Mock class, and in line 9 we create a Mock HttpResponseBase. Just creating the Mock does nothing more that create in memory an object that can respond to an interface, or (as in this case) an abstract base class.
In line 10 we setup the Mock HttpContextBase, so that when it receives a call to its Response property it will return the object represented by the Mock HttpResponse object.
In line 15 we set the HttpContext property of the presenter to this Mock HttpContextBase.
Now we have set up our test so we can test the view’s AdddVocabulary method (see Figure 4), which is designed to redirect to a new page when the AddVocabulary button is clicked.
Figure 4: The AddVocabulary method that Figure 3 is testing |
1: public void AddVocabulary(object sender, EventArgs e) 2: { 3: Response.Redirect(NavigateURL(TabId, _ 4: "CreateVocabulary", _ 5: String.Format("mid={0}", ModuleId))) 6: } |
So, in line 20 we raise the AddVocabulary event by calling the Raise method of the Mock view. This will cause the presenter’s event handler to be triggered and in line 23 we verify that this caused our Mock Response object to have its Redirect method called, with the expected url.
One of the advantages of a Mocking Framework like Moq is that it can be used to replace most types of Test Double. For example we can write the test using Dummy objects in Figure 2 to use Mocks (Figure 5).
Figure 5: The Example in Figure 2 Rewritten using Moq Instead of Dummies |
1: public void Constructor_Requires_Non_Null_VocabularyController() 2: { 3: //Arrange 4: Mock 5: Mock 6: 7: //Act, Assert 8: CreateVocabularyPresenter presenter = new CreateVocabularyPresenter(view.Object, 9: v, 10: scopeTypeController.Object); 11: 12: //.. 13: } |