As soon as you start unit-testing or test-driving your development, you’ll learn about test-doubles and how they can make your tests lightening-fast sooner or later. And if you set up a continuous integration process (which you should) and you have more than 5 unit tests, you’ll probably have to know about test-doubles sooner in stead of later :-) .

What is a test-double?

Gerard Meszaros defines a test-double as follows:

“Sometimes it is hard to test the system under test (SUT) because it depends on other components that cannot be used in the test environment. This could be because they aren’t available, they will not return the results needed for the test or because executing them would have undesirable side effects. In other cases, our test strategy requires us to have more control or visibility of the internal behavior of the SUT. When we are writing a test in which we cannot (or choose not to) use a real depended-on component (DOC), we can replace it with a Test Double. The Test Double doesn’t have to behave exactly like the real DOC; it merely has to provide the same API as the real one so that the SUT thinks it is the real one! “

The concept is very easy to understand, but if you’ve never heard of them, I assume that how a test-double looks like, is still a blurry thing. First I have to say you have to use them with caution and only when appropriated. Apart from that, there are several types of test-doubles. You can find a list of all types in Meszaros’ book and an enumeration of them here

Why use test-doubles?

I think I can summarize the need of a test-double in one line: Use a test-double to keep your tests focussed and fast. If you’re doing CI and TDD, you’ll have a very big test suite after a while, and it’s critical to keep it running in a few minutes. If you don’t, you’ll end up giving up CI, and you’ll loose the continous feedback it offers you.

If your SUT depends on a component that needs a lot of setup code or needs expensive resources, you don’t want to be doing all this in a simple test. Your SUT shouldn’t care how the component it depends on needs to be configured. If you are doing it, you’re writing integration or even acceptance tests to go through the whole system… That’s why replacing a DOC object with a fake, can come in very handy sometimes. Test your SUT in isolation, that’s the goal. The DOC-components will have tests of their own. And you’ll have integration tests on top of it all.

Expectations, verifications, and stuff like that

Before I get to mocks and stubs, you need to understand the expectation-verification thing.

First of all, a mock  or a stub, is just an object that looks like the real DOC, but is actually just a fake which you can use to pass the test, or record the calls your SUT makes to it. When using such a mock/stub, you can set expectations on it. An expectation, is a statement, in which you explicitly expect a call to a particular method or property, with particular parameters, and even a particular return value. After you’ve set the expectations you consider important*, you can verify that these calls actually took place, and thus verifying that your SUT is executing what you expected.

What is a stub?

A stub is an object that you use just to get your code passing. When you don’t really care about how the interaction with the DOC-object happens, you can use a stub to replace the real dependency. A stub can be an empty implementation or a so-called “dumb” implementation. In stead of performing a calculation, you could just return a fixed value.

When you use stubs using a mocking framework, it’s way easier than creating an extra class that can act as a stub for your test. How you do this exactly is for the upcoming post, but the good news is that you don’t need to manually code the stub.

What is a mock?

You’ll use mocks, when you really want to test the behavior of the system. You can set expectations on a mock, of methods/properties to be called with specified parameters and/or return arguments. The final part of the test is then always verification of the expectations that were set. If they were not satisified, your tests fails. This is especially interesting when you need to be completely sure that these were actually called. Just imagine an overly simplified profit calculator. You can never calculate your profits (or losses), if you havn’t calculated your revenues and expenses first, can you? Well, you could expect these are calculated first. (This is of course an overly simplified example for the sake of simplicity…)

What is a fake?

A fake is a class or method that’s implemented just for the sake of testing. It has the same goal as the other variations of test-doubles, replace the depend-on-component to avoid slow or unreliable tests. The classic example, is replacing a Repository that accesses the database, with an in memory repository, which just has a collection of objects it uses to return. That way you’ve got data that you can use to test the SUT on, without the overhead of communication with expensive or external components.

The database is just an example, you can perfectly use a fake object, to hide a complex processing of some kind, and just return the data you need to continue (which would be returned from the complex processing in production code). It will make your tests focus better on the SUT, and they will be a lot faster.


It’s almost impossible to unit test without using mocking-techniques. Your tests can become extremely slow when you have a lot of them, and the continuous feedback loop is lost.
Mocking is a very powerful technique, but beware of misusing it. I actually try to avoid mocks. Just think: Do I need to verify how my SUT interacts with the DOC? If not, don’t use a mock, use a stub. When using lots and lots of mocks, your tests can become brittle. Just imagine refactoring your DOC and breaking 20 tests. After looking into the problem, you notice that 17/20 tests broke because the expectations set on this DOC as a mock, aren’t fully correct anymore? That’s something you really should avoid. Keep your tests focused ;-) .

Recommended readings

Mocks aren’t stubs by Martin Fowler
Test doubles by Martin Fowler
xUnit Test Patterns by Gerard Meszaros (also check out the website)
Test Doubles: When (not) to use them by Davy Brion

I’ll continue this post with how you can use these types of test-doubles using a mocking framework like Rhino.Mocks as soon as I get the chance ;-) .