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.NET Unit Test Framework Comparison - Parallelism Granularity

2023/11/05 - Coenraad Stijne

Summary:

  • MSTest and NUnit support greater levels of Parallelism Granularity then xUnit.
  • When NUnit is configured to use Method, the documentation recommends changing the Test Class instantiation behavior from Single Instance to Instance Per Test for thread safety.

Personal Learnings:

  • MSTest and NUnit both support ctor/dispose pattern to setup/teardown tests, similar to xUnit.

Code Examples

Framework Overview

Framework Version Default Parallelism Maximum Parallelism
MSTest 3.1.1 None Method
NUnit 4.5.0 None Method
xUnit 2.6.1 Collection (class) Collection (class)

Note: When using Method level of parallelism, it is the developer's responsibility to ensure the tests are thread safe.

Configuring MSTest

ParallelizeAttribute Documentation

The Parallelize attribute can be set to configure the default strategy.

[assembly: Parallelize(Scope = ExecutionScope.MethodLevel)]

This will parallelize all methods, including DataRow and DynamicData.

✅ Thread Safety:

MSTest creates a new instance per test by default. Unless using static, fields/properties will be thread safe.

Configuring NUnit

ParallelizableAttribute Documentation
FixtureLifeCycle Documentation

The Parallelizable attribute can be set to configure the default strategy.
NUnit also recommends to set the FixtureLifeCycle attribute.

[assembly: Parallelizable(ParallelScope.Children)]
[assembly: FixtureLifeCycle(LifeCycle.InstancePerTestCase)]

This will parallelize all methods, including TestCase and TestCaseSource.

⚠️ Thread Safety:

The FixtureLifeCycle attribute was added so that NUnit will create a new instance of the test class for each test method executed. This is to ensure that fields/properties of the class will not be accessed by multiple tests at the same time.

Important Notes about FixtureLifeCycle:

  • When using LifeCycle.InstancePerTestCase, the OneTimeSetUp and OneTimeTearDown methods must be static, and each are only called once. This is required so that the setup or teardown methods do not access instance fields or properties that are reset for every test.

  • When using LifeCycle.InstancePerTestCase, a class's constructor will be called before every test is executed and IDisposable test fixtures will be disposed after the test is finished.

  • SetUp and TearDown methods are called before and after every test.

  • The Order attribute is respected.

Configuring xUnit

  • xUnit does not support method level parallelism natively
  • It is being considered for v3, see the Roadmap
  • There is a community solution, see xunit/xunit#1986 (comment)
  • Since it is not officially supported, it will not be covered in this post