Using NUnit's Test Selection Language in .NET Core

TLDR; In the next release of the NUnit3 VS Test Adapter, it will be possible to utilize filters written in NUnit’s native Test Selection Language.


With the release of .NET Core 3.1, I’ve been finding myself working on porting a number of code bases from Windows-only .NET Framework to this new exciting cross-platform world. My .NET Unit Testing Framework of choice has long been NUnit and a number of these code bases have utilized nunit to run continuous integration testing.

There is a lot of infrastructure in those CI processes that has been built around the nunit console runner. We specify output locations for the report xml via --work and --result, allowing the CI system to report on it. Since the system reports on the aggregate of test runs across environments and systems, we also utilize the Template Based Test Naming with --test-name-format to distinguish the same test across environments. Since a single test assembly might have tests that are only valid on a specific platform, over time a robust system of filters has developed using NUnit’s Test Selection Language and specified with --where.

Unfortunately, when it came time to start running some of these tests on non-Windows platforms, it was quickly discovered that the nunit console runner is a Windows-only application (since it is compiled for the legacy Windows Framework). Luckily, we had already added the NUnit3 VS Test Adapter to our test project in order to run tests from Visual Studio. This also allows us to use dotnet test to run the test project utilizing the NUnit test engine anywhere .NET Core can run. So, the next step was to figure out how to configure the test engine via dotnet test to act in the same way as it does with the where, test-name-format, work, and result command line options.

Configuring the NUnit Engine

As it turns out, the vstest framework has an extensible framework for configuring test runners using runsettings. The documentation for NUnit’s Adapter Settings shows a number of options that can be tweaked, and dotnet test allows us to set these on the command line. For instance given an existing command line of:

nunit3-console TestAssembly.dll --work=workDir --result=test-results\TestAssembly.xml --test-name-format=testEnvironment.{m}.{a}

You can get the same results with:

dotnet vstest TestAssembly.dll -- NUnit.WorkDirectory=workDir NUnit.TestOutputXml=test-results NUnit.DefaultTestNamePattern=testEnvironment.{m}.{a}

We use dotnet vstest in this example in order to operate on the pre-built test assembly. Passing runsettings to dotnet test to operate on a test project works exactly the same way.

Note that the result option differs, this is because the console runner allows specification of the filename for the output, while the runsettings only allow specification of the output directory (the filename is [TestAssembly].xml), so the two examples are equivalent.

The Filtering Problem

But… what about where? This is where things got a bit tricker. As it turns out, vstest itself already has a cross-framework filtering language that supports NUnit. Unfortunately, the syntax is completely different from NUnit’s native Test Selection Language. Perhaps one could try to translate between NUnit’s TSL and native VSTest filters, but that seems like adding a needless level of complexity. It’s also not clear that the VSTest filter language even supports all of the same features as NUnit’s Test Selection Language. Plus, we can already pass other computed arguments mostly unchanged between nunit3-console and dotnet test, why should where be any different? The NUnit engine used by both the console runner and the vstest adapter supports the Test Selection Language, why not just expose the functionality the same way?

The Solution

Clearly something had to be done, so I rolled up my sleeves and dug into the NUnit engine and vstest adapter code to see what I could do. A couple days of hacking later, and I submitted a pull request adding just this functionality to the NUnit VS Adapter Settings. Any existing where argument can be specified in the runsettings and it should work exactly the same as if it were passed to the console runner’s command line. This allows us to take an existing invocation of the console runner like:

nunit3-console TestAssembly.dll --where="cat == SomeCategory or method == SomeMethodName or namespace == My.Name.Space or name == 'TestMethod(5)'"

and pass the same filter unmodified to the test runner on .NET Core like:

dotnet test -- NUnit.Where="cat == SomeCategory or method == SomeMethodName or namespace == My.Name.Space or name == 'TestMethod(5)'"

And the exact same subset of tests should run.

Filtering can also be configured in a .runsettings file like so:

        <Where>cat == SomeCategory or method == SomeMethodName or namespace == My.Name.Space or name == 'TestMethod(5)'</Where>

And passed to dotnet test -s [filename].runsettings, dotnet vstest --settings:[filename].runsettings, or vstest.console.exe /settings:[filename].runsettings


There are a number of ways to configure the NUnit Test engine when running tests, using either the console runner or runsettings with vstest. In the next release of the vstest adapter, it will be possible to pass a NUnit Test Selection Language filter to the test engine using a NUnit.Where runsetting. This will allow filters that work with the existing console runner to be passed, unmodified, to the dotnet test command line.

Overtime, I would suspect more options and features to be added to the VS Test Adapter as it’s the most natural way to run NUnit tests under .NET Core.