AssemblyInitialize, AssemblyCleanup and Sharing State Between Test Classes in XUnit

We have covered quite a bit in this series on migrating from MSTest to XUnit and we have not even got to the coolest bit yet; data-driven theories. If that is what you are waiting for, you will have to wait a little longer. Before we get there, I want to cover one last piece of test initialization as provided in MSTest by the AssemblyInitialize and AssemblyCleanup attributes.

As we saw in previous posts, we can use the test class constructor and Dispose() for TestInitialize and TestCleanup, and IClassFixture<T> and fixture classes for ClassInitialize and ClassCleanup.  For the assembly equivalents, we use collections and the ICollectionFixture<T> interface.

A collection is defined by a set of test classes and a collection definition. A test class is designated as being part of a specific collection by decorating the class with the Collection attribute, which provides the collection name. A corresponding class decorated with the CollectionDefinition attribute should also exist as this is where any collection fixtures are defined. All classes that share the same collection name will share the collection fixtures from which the definition class derives.

The example code above shows a collection definition with two fixtures and two test classes defined as part of that collection. Note how the fixtures control initialization and cleanup using constructors and IDisposable 1 . We can modify those classes to reference the collection fixtures just as we did with class-level fixtures; by referencing the fixture in the constructor arguments as shown here.

I really like this approach over the attributed static methods of MSTest. This seems to more easily support code reuse and makes intentions much clearer, separating the concerns of tests (defined in the test class) from fixtures (defined by the fixture types). The downside is that fixture types do not get access to the ITestOutputHelper interface so if you want your fixtures to output diagnostic information, you should consider a logging library like Common.Logging. Also, your fixture types must be in the same assembly as your tests. Of course, that doesn't preclude the fixtures from using types outside of the assembly, so you can always put shared implementation between test assemblies in some other class library.

And that brings our migration of shared initialization to a close. You can find more information on sharing context across tests on the xunit site. Next up, we will look at data-driven tests. Thank you for your time. If you have any questions, please leave a comment.


  1. A fixture type can used with IClassFixture<T> or ICollectionFixture<T>  

ClassInitialize, ClassCleanup, and Sharing Data Across Tests in XUnit2

So far in this series on migrating from MSTest to XUnit, we have looked at:

In this post, we will look at how we can share setup and cleanup code across tests in a test class in XUnit. MSTest allows you to define shared setup and cleanup code for an entire test class by using methods decorated with the ClassInitialize and ClassCleanup attributes. Unlike their counterparts, TestInitialize and TestCleanup, methods decorated with these class-level attributes are executed just once per class, rather than once per test in the class. Using these class-level attributes, we can execute code, generate fixture objects, and load test data that can be used across all tests in the class without having the overhead of repeating this for every test in the class. This is useful when that initialization or cleanup is expensive, such as creating a database connection, or loading several data files.

As we have seen so far, XUnit is light on decorating non-test methods with attributes, instead relying on language syntax that mirrors the purpose of the code. In the case of TestInitialize and TestCleanup, XUnit uses the test class constructor and IDisposable. It should come as no surprise that this pattern is also used when it comes to class-level initialization and cleanup.

IClassFixture<T>

There are two parts to shared initialization and cleanup in XUnit: declaring what shared items a test class uses, and referencing them within test methods.

To declare specific setup is required, a test class must be derived from IClassFixture<T> for each shared setup/cleanup. The T in IClassFixture<T> is the actual type responsible for the initialization and cleanup via its constructor and IDisposable implementation.

The XUnit test runner sees that your test class is deriving from IClassFixture<MyFixture> and ensures that an instance of MyFixture is created before your tests are run and disposed of when all the tests are completed. I really like this approach over the MSTest equivalent, as it moves the setup and initialization from being about the test class to being about the test fixture, the thing being setup. You can even have more than one fixture, so if you use two databases in your tests, you can have one fixture for each database and explicitly specify the use of each. It also means that you can set things that are supposed to be immutable for the duration of tests to be readonly and enforce that immutability. This is even clearer when referencing fixtures in tests.

As shown in the preceding example, to reference a test fixture in your test class methods, you just need to add a corresponding argument to the constructor and XUnit will inject the fixture. You can then use the fixture, and assign it or something obtained from it to a member variable of your class. Not only that, but you can mark that member as readonly and be explicit about what tests can and cannot do to your test state. Personally, this approach to shared initialization and cleanup feels much more intuitive. I can easily reuse my initialization and setup code without cluttering my test classes unnecessarily, and I can be explicit about the immutability of any shared state or setup.

And that is it; now you not only know how to share repeatable setup across tests (as provided by TestInitialize and TestCleanup in MSTest), but also how to do the same for setup across the whole test class (as MSTest does with ClassIntialize and ClassSetup).

But, what of AssemblyInitialize and AssemblyCleanup? Well, that's probably a good place to start in the next post. As always, you are welcome to leave a comment letting me know how you are liking this series on migrating to XUnit, or perhaps bringing up something that you'd like me to cover.