Composable fixtures support

[neistow]

Currently, the fixture requiring another fixture doesn't work, throwing an exception.

Xunit.Sdk.TestPipelineException
Class fixture type 'Playground.Tests.FixtureB' had one or more unresolved constructor arguments: FixtureA fixtureA
   at Xunit.v3.FixtureMappingManager.GetFixture(Type fixtureType) in /_/src/xunit.v3.core/Utility/FixtureMappingManager.cs:line 164
   at Xunit.v3.FixtureMappingManager.InitializeAsync(IReadOnlyCollection`1 fixtureTypes) in /_/src/xunit.v3.core/Utility/FixtureMappingManager.cs:line 224

public class FixtureA
{
    public string A { get; private set; }
    
    public FixtureA()
    {
        A = "Can";
    }
}

public class FixtureB : IClassFixture<FixtureA>
{
    public string B { get; private set; }
    
    public FixtureB(FixtureA fixtureA)
    {
        B = fixtureA.A + " we make this work?";
    }
}

public class MyTests : IClassFixture<FixtureB>
{
    private readonly FixtureB _fixtureB;

    public MyTests(FixtureB fixtureB)
    {
        _fixtureB = fixtureB;
    }

    [Fact]
    public void Test()
    {
        Assert.NotEmpty(_fixtureB.B);
    }
}

I'm trying to share and compose some functionality used across tests. The only sketchy way I found of doing this is using TestContext.Current.GetFixture, but this heavily relies on interface implementation order and just doesn't feel right.

public class FixtureA
{
    public string A { get; private set; }

    public FixtureA()
    {
        A = "Can";
    }
}

public class FixtureB : IAsyncLifetime
{
    public string B { get; private set; } = null!;

    public FixtureB()
    {
    }

    public async ValueTask InitializeAsync()
    {
        var fixtureA = await TestContext.Current.GetFixture<FixtureA>();
        ArgumentNullException.ThrowIfNull(fixtureA);

        B = fixtureA.A + " we make this work?";
    }

    public ValueTask DisposeAsync() => ValueTask.CompletedTask;
}

// will work ONLY if IClassFixture<FixtureA> is implemented before IClassFixture<FixtureB>
public class MyTests : IClassFixture<FixtureA>, IClassFixture<FixtureB>
{
    private readonly FixtureB _fixtureB;

    public MyTests(FixtureB fixtureB)
    {
        _fixtureB = fixtureB;
    }

    [Fact]
    public void Test()
    {
        Assert.NotEmpty(_fixtureB.B);
    }
}

Any chance it will be implemented in xUnit? Other language frameworks allow fixtures to request other fixtures (rstest, pytest) to form complex object initialization.

[bradwilson]

No, we do not support this, and there is currently no plan to support this.

[neistow]

@bradwilson Thanks for the answer. I’d like to try and add support for this. Does it align with the current library priorities?

[bradwilson]

I would be willing to look at a PR for this, yes. It's not currently a priority for me to work on, which is a great reason for someone else to do the work. 😂