Oskar Dudycz

Pragmatic about programming

How to set up a test matrix in XUnit?

2021-02-21 oskar dudyczTests


Each country has the go-to place for hiding from daily struggles. In Poland, we have Bieszczady. It’s a mountain range that’s also the wildest part of our country. You will find there: solitude, forests, wolves, bobcats and all that wild nature stuff. Beautiful place with a mysterious vibe similar to Twin Peaks. We have a running joke in Poland, “Fuck that! Let’s go breed sheep in Bieszczady!”. That was precisely the feeling I had while adding System.Text.Json support to Marten.

If you’re following me on Twitter, then you probably saw a few of my rants about that. I’ll spare you my frustration and summarise: it was an unpleasant surprise. STJ is not covering many basic scenarios, and its design decisions are at least debatable (e.g. the need to use attributes everywhere).

Still, we decided to support System.Text.Json, as:

  • the limited feature-set may not hit users that need only simple POCO,
  • it may give performance improvement,
  • Newtonsoft Json.NET seems not to be supported enough and might get obsolete soon.

Adding a new serialiser to the library like Marten is a challenge. Marten, thanks to Postgres JSON capabilities uses it as a document database and an event store. JSON serialisation and translation is a centrepiece of our actions.

We have a big set of unit and integration tests (a few thousand) to ensure that all edge cases are covered. We are taking the free CI/CD engines spinning to their limits.

  • GitHub Actions - to run against .NET Core 3.1,
  • AzureDevops - for .NET 5.

Both environments are running build matrix tests for Postgres versions ranging from 9.6 to 12. That’s a hellova test runs and permutations.

As I mentioned, JSON serialisation and mapping (e.g., constructing SQL queries from LINQ) are critical in Marten. I could take a similar approach as I did in adding NodaTime support - dedicated tests. However, I wanted to make sure that we can precisely know what’s working and what’s not. That’s why I decided to add matrix tests to run for both serialisers.

That appeared harder than I thought. Both AzureDevops and GitHub Actions have decent support for the build matrix. However, this approach is against the XUnit conventions. It’s recommended to have explicit test data. It is a reasonable approach, but as I described, sometimes you have bigger needs. Tuning a framework to your needs is always hard. It took me a lot of googling, and a few tries to achieve that. That’s why I decided to share the solution with you.

In the samples below, I’ll be using, for clarity, sample project: https://github.com/oskardudycz/XUnit.MatrixTests/. I extracted part responsible for setting up the matrix tests with XUnit.

If you prefer, you can check the full Marten’s Pull Request: https://github.com/JasperFx/marten/pull/1685.

The first case to solve was passing information about the selected serialiser type for the test suite run. XUnit doesn’t support any test settings file. That’s why I followed KISS and used a good old environment variable. Based on it, I’m setting the default serialiser. To do that, I created two classes:

Factory to be able to select default serialiser:

public enum SerializerType
    NewtonsoftJsonNet = 1,
    SystemTextJson = 2

public static class SerializerFactory
    public static SerializerType DefaultSerializerType { get; set; } = SerializerType.NewtonsoftJsonNet;

    public static ISerializer New(SerializerType? serializerType = null)
        serializerType ??= DefaultSerializerType;

        return serializerType switch
            SerializerType.NewtonsoftJsonNet => new NewtonsoftSerializer(),
            SerializerType.SystemTextJson => new SystemTextJsonSerializer(),
            _ => throw new ArgumentOutOfRangeException(nameof(serializerType), serializerType, null)

Test setting class that, based on the environment variable, gets the serialiser type.

public static class TestsSettings
    private static SerializerType? serializerType;

    public static SerializerType SerializerType
            if (serializerType.HasValue) 
                return serializerType.Value;
            var defaultSerializerEnv = Environment.GetEnvironmentVariable("DEFAULT_SERIALIZER");

            serializerType = Enum.TryParse(defaultSerializerEnv, out SerializerType parsedSerializerType)
                ? parsedSerializerType
                : SerializerType.NewtonsoftJsonNet;
            return serializerType.Value;

Having those classes, the only issue left was initialising the test settings automatically before the XUnit test run. It’s possible by creating your own XunitTestFramework.

using Weasel.Serialization;
using Xunit;
using Xunit.Abstractions;
using Xunit.Sdk;

[assembly: TestFramework("XUnit.MatrixTests.TestSetup", "XUnit.MatrixTests")]

namespace XUnit.MatrixTests
    public class TestSetup : XunitTestFramework
        public TestSetup(IMessageSink messageSink)
            SerializerFactory.DefaultSerializerType = TestsSettings.SerializerType;

        public new void Dispose()
            // Place tear down code here

You can do a lot of customisation there, but for our case, it was enough to connect the dots and set the default serialiser type. Besides deriving from XunitTestFramework it’s also needed to put the assembly attribute. Beware - you need to use the magical strings with the name of your tests project assembly and defined test framework name with a namespace. The wrong copy&paste can hurt a lot!

That’s all to have the XUnit build matrix!

Well, almost. I knew already that some of the test scenarios wouldn’t work. System.Text.Json is not working correctly with the anonymous and dynamic types, classes hierarchies etc. I wanted to have an option to mark some tests as only supported for Newtonsoft. I used the pattern that we already had for skipping features unsupported in the older Postgres versions (e.g. full-text search below version 10). I implemented the custom XUnit Fact for that with information about the selected serialiser.

[XunitTestCaseDiscoverer("XUnit.MatrixTests.Extras.SerializerTargetedFactDiscoverer", "XUnit.MatrixTests")]
public sealed class SerializerTypeTargetedFact: FactAttribute
    public SerializerType RunFor { get; set; }

Besides that, I needed to implement a custom fact discoverer to skip the test if the serialiser used in the test run is different than selected in SerializerTypeTargetedFact.

public sealed class SerializerTargetedFactDiscoverer: FactDiscoverer
    private readonly SerializerType serializerType;

    public SerializerTargetedFactDiscoverer(IMessageSink diagnosticMessageSink): base(diagnosticMessageSink)
        serializerType = TestsSettings.SerializerType;

    public override IEnumerable<IXunitTestCase> Discover(ITestFrameworkDiscoveryOptions discoveryOptions, ITestMethod testMethod,
        IAttributeInfo factAttribute)
        var runForSerializer = factAttribute.GetNamedArgument<SerializerType?>(nameof(SerializerTypeTargetedFact.RunFor));
        if (runForSerializer != null && runForSerializer != serializerType)
            yield return new TestCaseSkippedDueToSerializerSupport($"Test skipped as it cannot be run for {serializerType} ", DiagnosticMessageSink, discoveryOptions.MethodDisplayOrDefault(), discoveryOptions.MethodDisplayOptionsOrDefault(), testMethod);
            yield break;
        yield return CreateTestCase(discoveryOptions, testMethod, factAttribute);

    internal sealed class TestCaseSkippedDueToSerializerSupport: XunitTestCase
        [Obsolete("Called by the de-serializer", true)]
        public TestCaseSkippedDueToSerializerSupport()

        public TestCaseSkippedDueToSerializerSupport(string skipReason, IMessageSink diagnosticMessageSink, TestMethodDisplay defaultMethodDisplay, TestMethodDisplayOptions defaultMethodDisplayOptions, ITestMethod testMethod, object[] testMethodArguments = null) : base(diagnosticMessageSink, defaultMethodDisplay, defaultMethodDisplayOptions, testMethod, testMethodArguments)
            SkipReason = skipReason;

Then I could use it as:

[SerializerTypeTargetedFact(RunFor = SerializerType.NewtonsoftJsonNet)]
 public void TestWithCustomFact()
     var session = new DocumentSession();

     var doc = session.Load<dynamic>("1");
     Assert.Equal("1", (string)doc.Id);
     Assert.Equal("test", (string)doc.Name);

You can implement the custom XUnit Theory. But, I’ll let you check it directly on GitHub: https://github.com/oskardudycz/XUnit.MatrixTests/blob/main/XUnit.MatrixTests/Extras/SerializerTypeTargetedTheory.cs.

As the final touch for this blog post, I’ll show you how to configure such a test matrix in GitHub Actions:

name: ASP.NET Core CI

on: [push]

      runs-on: ubuntu-latest
          serializer: [Newtonsoft, SystemTextJson]

        - name: Check Out Repo
          uses: actions/checkout@v1

        - name: Setup .NET Core
          uses: actions/setup-dotnet@v1
            dotnet-version: '5.0.100'

        - name: Build with dotnet
          run: dotnet build --configuration Release 
        - name: Test with dotnet
          run: dotnet test --configuration Release 
            DEFAULT_SERIALIZER: ${{ matrix.serializer }}

Yes, it’s so simple. You need to define the set of matrix variables (serializer: [Newtonsoft, SystemTextJson]). And pass them to the build step environment variable.

Magic! Smoke and Mirrors!

Breeding sheep in Bieszczady is a decent idea. But programming still can be fun. I hope that this post will help you and save you some of the head-banging-on-desk experience.



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