Why I stopped worrying about test setups by using AutoFixture

Tim Deschryver

The next project I'm setting up will use AutoFixture from the start to set up the "arrange" part of a test and here is why.

When a new project is in the start-up phase all models are relatively small, simple, and do not have a lot of relations. This makes the tests easy to arrange and also easy to read. Because the team is mostly spending their time implementing new features, there's a lot of focus on writing new tests for these new features. For every feature, there are one or more tests that go with that feature.

Throughout the lifetime of the project, these models keep on growing, making it harder to write new tests, sadly the existing tests are starting to feel like a chore to maintain.

Once the basic functionality is developed, the team is also maintaining the existing codebase and thus new features (from scratch) are less developed. Instead, the behavior of the application is extended step by step. These smaller changes feel to have a lower impact, resulting that fewer tests are written. The existing tests also receive less attention.

Because of the current state of the project, a failing test only receives attention when it's failing. With every addition, things are getting worse.

At some point, arranging a test might even be the trickiest part of the test. Worse, when the requirements change, we might lose the confidence the tests were giving us previously because they're not kept up to date.
The team is forced to change the application's code and the test code in one single go. When this point is reached, it's usually too late and the team wonders when things were starting to go downhill.

From my experience, a good test setup is a crucial factor that makes a test easy and fast to write (and to maintain), otherwise, a bad test setup makes the tests feel like an unpleasant chore to maintain which is a loss of time. This is caused because the tests are too much coupled to the application's codebase. Another point that makes unit tests hard to maintain are those tests that are testing implementation details, which is why I prefer to write integration tests but that's not what this blog post is about.

Since a model is always changing, ideally want our test setups (fixtures) to be up-to-date at all times, but that shouldn't be the concern of the tests. As a remedy, we can use the AutoFixture library.


Write maintainable unit tests, faster. AutoFixture makes it easier for developers to do Test-Driven Development by automating non-relevant Test Fixture Setup, allowing the Test Developer to focus on the essentials of each test case.

The Problem with Test Setups link

Let's take a look at an example. In the example, a team starts with a simple customer object that we keep enhancing to answer the requirement changes.

After the first iteration, the customer has an id and a name. Nice and simple.

Since this is just the beginning, creating a fixture is easy and thus is the customer manually created in the test.

Of course, the customers' system can also update and delete customers so this test setup is copy-pasted across multiple tests.

We're a couple days further and the customer needs to be extended with an address.

To accommodate this change, the team has to go back to all of the created tests and provide an address to every customer. Yikes.

After this abrupt change, the team decides that they won't be bitten by future changes to the model. That's why they resort to battle-tested design patterns to create their test fixtures. For example, by using a factory method or by using the builder pattern to create new instances of the customer.

After the team refactored the tests, they are confident that their tests are robust against future model changes. What they don't realize is that this code needs to be maintained and that it needs to reflect model changes. Sadly, this isn't always as simple as it seems.

Let's make another change that affects the customer model. This time, a collection of orders are added to the customer's model.

Because the orders are optional, the developer forgets to update the test fixtures and thus these are out of sync. Luckily, another team member notices this and asks if the fixtures can be updated. The developer now has to make a choice, should all customers have orders by default, should there be two different implementations, ...?

While this example is kept simple, I hope you get the point I'm trying to make here. There's even a chance that you've already experienced this in a real project. Maintaining test fixtures can be hard and time-consuming (having discussions with team members, having to write the code). Besides that, the test setup alone could take up a whole lot of lines of code.

AutoFixture link

Now that we've seen the problem, let's see how AutoFixture provides a solution.

If you're not familiar with AutoFixture, the short version (as the name implies) is that the library automagically creates test fixture instances (known as specimens) for your objects with random test data. To be honest "random data" is not the correct term to use here, Constrained Non-Deterministic data with Explicit Expectations would be a much better term. For example, you will notice that string properties have the property name postfixed with a GUID as value, that numbers are always positive, and there are always 3 items in a collection.


Values are carefully generated to stay far away from any boundary conditions that may cause the SUT to behave differently in each test run.

Let's refactor the customer tests by using some of AutoFixture's features. To create the customer with AutoFixture, the first thing to do is to create a new Fixture instance. Next, you can use the Create method to create an instance of a generic, in our example, a customer.

This gives us the following customer's value, which does change every time the test is run. As you can see below, all properties of the Customer object have a random value, even the nested Address and Orders objects.

To completely remove the arrange part of the test, we can install the package AutoFixture for XUnit or AutoFixture for NUnit. These packages provide an AutoData attribute that creates the fixtures for us. To use them, change the test from a Fact to a Theory and inject a customer as a parameter.

We end up with a test that is clean and only contains the important parts. With a single blink of an eye, we can read the test.

But Tim, how do you test specific requirements when everything is non-deterministic?

Influence test generation link

The answer to that question doesn't have to be complicated. In fact, AutoFixture is designed so that 80% of created specimens don't need a manual touch from a developer. For those cases where it's needed, you can simply overwrite the generated object in the test.

This keeps things simple and it's a good way to become familiar with AutoFixture. On the other hand, it doesn't provide a complete answer to the problem. While the current implementation keeps testing models in sync with the application code and is fast to set up, it still doesn't keep the tests DRY.

Customize fixtures link

Overwriting properties in a single test is perfect for one-off tests but for it to be a sustainable solution, we must take a deeper dive into the API of AutoFixture.

With Customizations, it becomes possible to hook into the generation process of an object. We're going to continue to use the XUnit's parameterized tests, and create an AutoDataAttribute to provide the parameters to the test. If you're not a fan of parameterized tests, you can also extend the fixture inline in a similar way.

To customize the customer from our example, we use the builder API from AutoFixture to give the important properties a specific value. Use the With method for each property that has to be overwritten, and assign it a value. In the example below, we create a NoOrdersDataAttribute that contains a customer that has 0 orders.

A customization is composable, therefore we can refactor the above snippet into a reusable customization. To accomplish this, we create a custom customization class that implements the ICustomization interface.

Next, we can use NoOrdersCustomization to customize the fixture by replacing the inline customization with the new NoOrdersCustomization class.

Finally, this attribute can be used in the tests to create new customers that have no orders.

Just with everything, it's important to give a proper name to your customizations and attributes.

Specimens link

Another powerful way to hook into AutoFixture's generation, are specimens. A specimen allows for a more generic approach to impact the generated data.

For example, if we want to prefix all product SKU's, we can implement this as a specimen.

To do so, create a new class and implement the ISpecimenBuilder interface. Every generated object passes through this builder. Via reflection, we're looking for properties with the name ProductSku. If we find that property, the desired value is returned, otherwise, a new NoSpecimen instance is returned. By doing this, AutoFixtures knows if the property already has a user-defined value or if it needs to generate a value for that property.

The last thing to do is to add the ProductSkuSpecimenBuilder to the fixture created in the NoOrdersDataAttribute.

Extra Tips link

Generating multiple models link

Some tests need multiple generated instances, therefore, a parametrized test can have multiple parameters.

Composite Customizations link

Create a CompositeCustomization to combine multiple customizations into one.

Conclusion link

Because AutoFixture does all of the heavy liftings for me, I stopped overthinking my test setups and I have more time to implement new features.

Most of the properties of an object don't affect the business logic, thus these can just be ignored during the test setup. This drastically reduced the noise of my tests, and I was able to remove a lot of test setup code. Because there's a low coupling between the application code and test code, it also means that I don't have to go back to my tests every time a model is refactored.

The default generated model is perfect for quick and simple test cases, while the model can still be overwritten in a clean way for specific test cases. The customizations are a great way to create default prerequisites of a model in complex domains.

Since there's only a little bit of documentation that can be found in the AutoFixture repository, I really liked and learned a lot from the blog posts written by Mark Seemann, who is also the creator of AutoFixture. So definitely make sure to check those out!

Lastly, here are some highlights of why I think you should be using AutoFixture as a test fixture builder:

Incoming links

Outgoing links

Feel free to update this blog post on GitHub, thanks in advance!

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