Hard-Coded Mock Objects vs Mocking Framework

Question

I\'m curious as to what method people like to use for mocking and why. The two methods that I know of are using hard coded mock objects and a mocking framework. To

Answer 1

I think the choice between writing dummy objects by hand or by using a framework depends a lot upon the types of components that you are testing.

If it is part of the contract for the component under test to communicate with its collaborators following a precise protocol, then instrumented dummy objects ("Mocks") are just the thing to use. It is frequently much easier to test such protocols using a mocking framework than by hand-coding. Consider a component that is required to open a repository, perform some reads and writes in a prescribed order, and then close the repository -- even in the face of an exception. A mocking framework would make it easier to set up all of the necessary tests. Applications related to telecommunications and process control (to pick a couple of random examples) are full of components that need to be tested in this fashion.

On the other hand, many components in general business applications have no particular constraints on how they communicate with their collaborators. Consider a component that performs some kind of analysis of, say, university course loads. The component needs to retrieve instructor, student and course information from a repository. But it does not matter what order it retrieves the data: instructor-student-course, student-course-instructor, all-at-once, or whatever. There is no need to test for and enforce a data access pattern. Indeed, it would likely be harmful to test that pattern as it would be demanding a particular implementation unnecessarily. In this context, simple uninstrumented dummy objects ("Stubs") are adequate and a mocking framework is probably overkill.

I should point out that even when stubbing, a framework can still make your life a lot easier. One doesn't always have the luxury of dictating the signatures of one's collaborators. Imagine unit-testing a component that is required to process data retrieved from a thick interface like IDataReader or ResultSet. Hand-stubbing such interfaces is unpleasant at best -- especially if the component under test only actually uses three of the umpteen methods in the interface.

For me, the projects that have required mocking frameworks were almost invariably of a systems-programming nature (e.g. database or web infrastructure projects, or low-level plumbing in a business application). For applications-programming projects, my experience has been that there were few mocks in sight.

Given that we always strive to hide the messy low-level infrastructure details as much as possible, it would seem that we should aim to have the simple stubs far outnumber the mocks.

Answer 2

I tend to write stubs and mocks by hand, first. Then if it can be easily expressed using a mock object framework, I rewrite it so that I have less code to maintain.

Answer 3

I have been writing them by hand. I was having trouble using Moq, but then I read TDD: Introduction to Moq, and I think I get what they say about classical vs. mockist approaches now. I'll be giving Moq another try this evening, and I think understanding the "mockist" approach will give me what I need to make Moq work better for me.

Answer 4

A mocked interface can have different outputs per test - One test you may have a method return null, another test has the method return an object, another test has the method throw an exception. This is all configured in the unit test, whereas your version would require several hand-written objects.

Psuedocode:

//Unit Test One

MockObject.Expect(m => m.GetData()).Return(null);

//Unit Test Two

MockObject.Expect(m => m.GetData()).Return(new MyClass());

//Unit Test Three

MockObject.Expect(m => m.GetData()).ThrowException();

Answer 5

Some distinguish between mocks and stubs. A mock object may verify that it has been interacted with in the expected way. A mocking framework can make it easy to generate mocks and stubs.

In your example, you've stubbed out a single method in an interface. Consider an interface with n methods, where n can change over time. A hand-stubbed implementation may require more code and more maintenance.

Answer 6

That's not a Mock, it's a Stub. For stubbing, your example is perfectly acceptable.

From Martin Fowler:

Mocks are what we are talking about here: objects pre-programmed with expectations which form a specification of the calls they are expected to receive.

When you're mocking something, you usually call a "Verify" method.

Look at this for the diff between Mocks and Stubs http://martinfowler.com/articles/mocksArentStubs.html