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Uniqueness of xunit collection fixtures
#1 Posted : Wednesday, December 9, 2020 7:19:12 PM(UTC)
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Hi there,

I am currently facing a problem with an ICollectionFixture in xunit and I am not sure if this is an issue in the NCrunch engine or if I am simply missing some magic configuration I did not discover yet.

The fixture is shared with all test classes through a collection, so I would expect it to behave like a singleton. When executing with Visual Studio, it behaves as I would expect (only one instance of the fixture is instantiated throughout test execution).
But running with NCrunch, the fixture gets instantiated multiple times*). Parallel test execution is explicitly disabled. I could strip this behavior down to a project with only the following code:


public class UnitTest1
    private readonly Fixture fixture;

    public UnitTest1(Fixture fixture)
        this.fixture = fixture;

    public void Test1()


public class UnitTest2
    private readonly Fixture fixture;

    public UnitTest2(Fixture fixture)
        this.fixture = fixture;

    public void Test2()


public class Fixture
    private static int counter = 0;
    public Fixture()
        int newCount = Interlocked.Increment(ref counter);
        if (newCount > 1)
            throw new Exception($"{newCount} instances? WTF???");

public class TestCollectionDefinition : ICollectionFixture<Fixture>

Thanks for any support,

*) More details on NCrunch behavior based on the sample code:
- When starting all tests in Debug, it works.
- When all tests have the same state before execution, the execution works too.
- When one test is red, one is green, the Exception in the fixture's ctor is thrown on that test that previously was green. Further executing all tests lets both facts toggle between success and failure, meaning: The test, which was in failing state gets the first fixture instance, the succeeding test gets the second fixture instance. Feels like it depends if there are multiple execution queues/priorities of the tests?

P.S.: NCrunch version is
#2 Posted : Wednesday, December 9, 2020 11:01:04 PM(UTC)
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Hi, thanks for posting.

This is probably happening because of NCrunch's batching. NCrunch separates tests into tasks, which can be seen in the processing queue. Each task results into a new call into the test framework, with instructions to run a set of tests. In theory this could result in a test collection being instantiated multiple times in a given test process, as the test process is used for multiple test runs (see test runner re-use).

You have a couple of options here. Either use static variables to determine when the code in your singleton is being re-executed (thus ensuring it isn't), or try marking the tests involved with NCrunch.Framework.IsolatedAttribute.
1 user thanked Remco for this useful post.
jbartel on 12/14/2020(UTC)
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