dotnet-xunit

xUnit v3 testing framework features for .NET. Covers [Fact] and [Theory] attributes, test fixtures (IClassFixture, ICollectionFixture), parallel execution configuration, IAsyncLifetime for async setup/teardown, custom assertions, and xUnit analyzers. Includes v2 compatibility notes where behavior differs.

Version assumptions: xUnit v3 primary (.NET 8.0+ baseline). Where v3 behavior differs from v2, compatibility notes are provided inline. xUnit v2 remains widely used; many projects will encounter both versions during migration.

Out of scope: Test project scaffolding (creating xUnit projects, package references, Directory.Build.props) is owned by [skill:dotnet-add-testing]. Testing strategy and test type decisions are covered by [skill:dotnet-testing-strategy]. Integration testing patterns (WebApplicationFactory, Testcontainers) are covered by [skill:dotnet-integration-testing].

Prerequisites: Test project already scaffolded via [skill:dotnet-add-testing] with xUnit packages referenced. Run [skill:dotnet-version-detection] to confirm .NET 8.0+ baseline for xUnit v3 support.

Cross-references: [skill:dotnet-testing-strategy] for deciding what to test and how, [skill:dotnet-integration-testing] for combining xUnit with WebApplicationFactory and Testcontainers.

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xUnit v3 vs v2: Key Changes

Feature	xUnit v2	xUnit v3
Package	xunit (2.x)	xunit.v3
Runner	xunit.runner.visualstudio	xunit.runner.visualstudio (3.x)
Async lifecycle	IAsyncLifetime	IAsyncLifetime (now returns ValueTask)
Assert package	Bundled	Separate xunit.v3.assert (or xunit.v3.assert.source for extensibility)
Parallelism default	Per-collection	Per-collection (same, but configurable per-assembly)
Timeout	Timeout property on [Fact] and [Theory]	Timeout property on [Fact] and [Theory] (unchanged)
Test output	ITestOutputHelper	ITestOutputHelper (unchanged)
[ClassData]	Returns IEnumerable<object[]>	Returns IEnumerable<TheoryDataRow<T>> (strongly typed)
[MemberData]	Returns IEnumerable<object[]>	Supports TheoryData<T> and TheoryDataRow<T>
Assertion messages	Optional string parameter on Assert methods	Removed in favor of custom assertions (v3.0); use Assert.Fail() for explicit messages

v2 compatibility note: If migrating from v2, replace xunit package with xunit.v3. Most [Fact] and [Theory] tests work without changes. The primary migration effort is in IAsyncLifetime (return type changes to ValueTask), [ClassData] (strongly typed row format), and removed assertion message parameters.

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Facts and Theories

[Fact] -- Single Test Case

Use [Fact] for tests with no parameters:

public class DiscountCalculatorTests
{
    [Fact]
    public void Apply_NegativePercentage_ThrowsArgumentOutOfRangeException()
    {
        var calculator = new DiscountCalculator();

        var ex = Assert.Throws<ArgumentOutOfRangeException>(
            () => calculator.Apply(100m, percentage: -5));

        Assert.Equal("percentage", ex.ParamName);
    }

    [Fact]
    public async Task ApplyAsync_ValidDiscount_ReturnsDiscountedPrice()
    {
        var calculator = new DiscountCalculator();

        var result = await calculator.ApplyAsync(100m, percentage: 15);

        Assert.Equal(85m, result);
    }
}

[Theory] -- Parameterized Tests

Use [Theory] to run the same test logic with different inputs.

[InlineData]

Best for simple value types:

[Theory]
[InlineData(100, 10, 90)]      // 10% off 100 = 90
[InlineData(200, 25, 150)]     // 25% off 200 = 150
[InlineData(50, 0, 50)]        // 0% off = no change
[InlineData(100, 100, 0)]      // 100% off = 0
public void Apply_VariousInputs_ReturnsExpectedPrice(
    decimal price, decimal percentage, decimal expected)
{
    var calculator = new DiscountCalculator();

    var result = calculator.Apply(price, percentage);

    Assert.Equal(expected, result);
}

[MemberData] with TheoryData<T>

Best for complex data or shared datasets:

public class OrderValidatorTests
{
    public static TheoryData<Order, bool> ValidationCases => new()
    {
        { new Order { Items = [new("SKU-1", 1)], CustomerId = "C1" }, true },
        { new Order { Items = [], CustomerId = "C1" }, false },              // no items
        { new Order { Items = [new("SKU-1", 1)], CustomerId = "" }, false }, // no customer
    };

    [Theory]
    [MemberData(nameof(ValidationCases))]
    public void IsValid_VariousOrders_ReturnsExpected(Order order, bool expected)
    {
        var validator = new OrderValidator();

        var result = validator.IsValid(order);

        Assert.Equal(expected, result);
    }
}

[ClassData]

Best for data shared across multiple test classes:

// xUnit v3: use TheoryDataRow<T> for strongly-typed rows
public class CurrencyConversionData : IEnumerable<TheoryDataRow<string, string, decimal>>
{
    public IEnumerator<TheoryDataRow<string, string, decimal>> GetEnumerator()
    {
        yield return new("USD", "EUR", 0.92m);
        yield return new("GBP", "USD", 1.27m);
        yield return new("EUR", "GBP", 0.86m);
    }

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

// xUnit v2 compatibility: v2 uses IEnumerable<object[]> instead of TheoryDataRow<T>
// public class CurrencyConversionData : IEnumerable<object[]>
// {
//     public IEnumerator<object[]> GetEnumerator()
//     {
//         yield return new object[] { "USD", "EUR", 0.92m };
//     }
//     IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
// }

[Theory]
[ClassData(typeof(CurrencyConversionData))]
public void Convert_KnownPairs_ReturnsExpectedRate(
    string from, string to, decimal expectedRate)
{
    var converter = new CurrencyConverter();

    var rate = converter.GetRate(from, to);

    Assert.Equal(expectedRate, rate, precision: 2);
}

---

Fixtures: Shared Setup and Teardown

Fixtures provide shared, expensive resources across tests while maintaining test isolation.

IClassFixture<T> -- Shared Per Test Class

Use when multiple tests in the same class share an expensive resource (database connection, configuration):

public class DatabaseFixture : IAsyncLifetime
{
    public string ConnectionString { get; private set; } = "";

    public ValueTask InitializeAsync()
    {
        // xUnit v3: returns ValueTask (v2 returns Task)
        ConnectionString = $"Host=localhost;Database=test_{Guid.NewGuid():N}";
        // Create database, run migrations, etc.
        return ValueTask.CompletedTask;
    }

    public ValueTask DisposeAsync()
    {
        // xUnit v3: returns ValueTask (v2 returns Task)
        // Drop database
        return ValueTask.CompletedTask;
    }
}

public class OrderRepositoryTests : IClassFixture<DatabaseFixture>
{
    private readonly DatabaseFixture _db;

    public OrderRepositoryTests(DatabaseFixture db)
    {
        _db = db;
        // Each test gets the shared database fixture
    }

    [Fact]
    public async Task GetById_ExistingOrder_ReturnsOrder()
    {
        var repo = new OrderRepository(_db.ConnectionString);

        var result = await repo.GetByIdAsync(KnownOrderId);

        Assert.NotNull(result);
    }
}

v2 compatibility note: In xUnit v2, IAsyncLifetime.InitializeAsync() and DisposeAsync() return Task. In v3, they return ValueTask. When migrating, change the return types accordingly.

ICollectionFixture<T> -- Shared Across Test Classes

Use when multiple test classes need the same expensive resource:

// 1. Define the collection
[CollectionDefinition("Database")]
public class DatabaseCollection : ICollectionFixture<DatabaseFixture>
{
    // This class has no code -- it is a marker for the collection
}

// 2. Use in test classes
[Collection("Database")]
public class OrderRepositoryTests
{
    private readonly DatabaseFixture _db;

    public OrderRepositoryTests(DatabaseFixture db)
    {
        _db = db;
    }

    [Fact]
    public async Task Insert_ValidOrder_Persists()
    {
        // Uses the shared database fixture
    }
}

[Collection("Database")]
public class CustomerRepositoryTests
{
    private readonly DatabaseFixture _db;

    public CustomerRepositoryTests(DatabaseFixture db)
    {
        _db = db;
    }
}

IAsyncLifetime on Test Classes

For per-test async setup/teardown without a shared fixture:

public class FileProcessorTests : IAsyncLifetime
{
    private string _tempDir = "";

    public ValueTask InitializeAsync()
    {
        _tempDir = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
        Directory.CreateDirectory(_tempDir);
        return ValueTask.CompletedTask;
    }

    public ValueTask DisposeAsync()
    {
        if (Directory.Exists(_tempDir))
            Directory.Delete(_tempDir, recursive: true);
        return ValueTask.CompletedTask;
    }

    [Fact]
    public async Task Process_CsvFile_ExtractsRecords()
    {
        var filePath = Path.Combine(_tempDir, "data.csv");
        await File.WriteAllTextAsync(filePath, "Name,Age\nAlice,30\nBob,25");

        var processor = new FileProcessor();
        var records = await processor.ProcessAsync(filePath);

        Assert.Equal(2, records.Count);
    }
}

---

Parallel Execution

Default Behavior

xUnit runs test classes within a collection sequentially but runs different collections in parallel. Each test class without an explicit [Collection] attribute is its own implicit collection, so by default test classes run in parallel.

Controlling Parallelism

Disable Parallelism for Specific Tests

Place tests that share mutable state in the same collection:

[CollectionDefinition("Sequential", DisableParallelization = true)]
public class SequentialCollection { }

[Collection("Sequential")]
public class StatefulServiceTests
{
    // These tests run sequentially within this collection
}

Assembly-Level Configuration

Create xunit.runner.json in the test project root:

{
    "$schema": "https://xunit.net/schema/current/xunit.runner.schema.json",
    "parallelizeAssembly": false,
    "parallelizeTestCollections": true,
    "maxParallelThreads": 4
}

Ensure it is copied to output:

<ItemGroup>
  <Content Include="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
</ItemGroup>

v2 compatibility note: In v2, configuration was via xunit.runner.json or assembly attributes. v3 retains xunit.runner.json support with the same property names.

---

Test Output

ITestOutputHelper

Capture diagnostic output that appears in test results:

public class DiagnosticTests
{
    private readonly ITestOutputHelper _output;

    public DiagnosticTests(ITestOutputHelper output)
    {
        _output = output;
    }

    [Fact]
    public async Task ProcessBatch_LargeDataset_CompletesWithinTimeout()
    {
        var sw = Stopwatch.StartNew();

        var result = await processor.ProcessBatchAsync(largeDataset);

        sw.Stop();
        _output.WriteLine($"Processed {result.Count} items in {sw.ElapsedMilliseconds}ms");
        Assert.True(sw.Elapsed < TimeSpan.FromSeconds(5));
    }
}

Integrating with ILogger

Bridge xUnit output to Microsoft.Extensions.Logging for integration tests:

// NuGet: Microsoft.Extensions.Logging (for LoggerFactory)
// + a logging provider that writes to ITestOutputHelper
// Common approach: use a simple adapter
public class XunitLoggerProvider : ILoggerProvider
{
    private readonly ITestOutputHelper _output;

    public XunitLoggerProvider(ITestOutputHelper output) => _output = output;

    public ILogger CreateLogger(string categoryName) =>
        new XunitLogger(_output, categoryName);

    public void Dispose() { }
}

public class XunitLogger(ITestOutputHelper output, string category) : ILogger
{
    public IDisposable? BeginScope<TState>(TState state) where TState : notnull => null;
    public bool IsEnabled(LogLevel logLevel) => true;

    public void Log<TState>(LogLevel logLevel, EventId eventId, TState state,
        Exception? exception, Func<TState, Exception?, string> formatter)
    {
        output.WriteLine($"[{logLevel}] {category}: {formatter(state, exception)}");
        if (exception is not null)
            output.WriteLine(exception.ToString());
    }
}

---

Custom Assertions

Extending Assert with Custom Methods

Create domain-specific assertions for cleaner test code:

public static class OrderAssert
{
    public static void HasStatus(Order order, OrderStatus expected)
    {
        Assert.NotNull(order);
        if (order.Status != expected)
        {
            throw Xunit.Sdk.EqualException.ForMismatchedValues(
                expected, order.Status);
        }
    }

    public static void ContainsItem(Order order, string sku, int quantity)
    {
        Assert.NotNull(order);
        var item = Assert.Single(order.Items, i => i.Sku == sku);
        Assert.Equal(quantity, item.Quantity);
    }
}

// Usage
[Fact]
public void Complete_ValidOrder_SetsCompletedStatus()
{
    var order = new Order();
    order.Complete();

    OrderAssert.HasStatus(order, OrderStatus.Completed);
}

Using Assert.Multiple (xUnit v3)

Group related assertions so all are evaluated even if one fails:

[Fact]
public void CreateOrder_ValidRequest_SetsAllProperties()
{
    var order = OrderFactory.Create(request);

    Assert.Multiple(
        () => Assert.Equal("cust-123", order.CustomerId),
        () => Assert.Equal(OrderStatus.Pending, order.Status),
        () => Assert.NotEqual(Guid.Empty, order.Id),
        () => Assert.NotEmpty(order.Items)
    );
}

v2 compatibility note: Assert.Multiple is new in xUnit v3. In v2, use separate assertions -- the test stops at the first failure.

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xUnit Analyzers

The xunit.analyzers package (included with xUnit v3) catches common test authoring mistakes at compile time.

Important Rules

Rule	Description	Severity
xUnit1004	Test methods should not be skipped	Info
xUnit1012	Null should not be used for value type parameters	Warning
xUnit1025	InlineData should be unique within a Theory	Warning
xUnit2000	Constants and literals should be the expected argument	Warning
xUnit2002	Do not use null check on value type	Warning
xUnit2007	Do not use typeof expression to check type	Warning
xUnit2013	Do not use equality check to check collection size	Warning
xUnit2017	Do not use Contains() to check if value exists in a set	Warning

Suppressing Specific Rules

In .editorconfig for test projects:

[tests/**.cs]
# Allow skipped tests during development
dotnet_diagnostic.xUnit1004.severity = suggestion

---

Key Principles

• One fact per [Fact], one concept per [Theory]. If a [Theory] tests fundamentally different scenarios, split into separate [Fact] methods.
• Use IClassFixture for expensive shared resources within a single test class. Use ICollectionFixture when multiple classes share the same resource.
• Do not disable parallelism globally. Instead, group tests that share mutable state into named collections.
• Use IAsyncLifetime for async setup/teardown instead of constructors and IDisposable. Constructors cannot be async, and IDisposable.Dispose() does not await.
• Keep test data close to the test. Prefer [InlineData] for simple cases. Use [MemberData] or [ClassData] only when data is complex or shared.
• Enable xUnit analyzers in all test projects. They catch common mistakes that lead to false-passing or flaky tests.

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Agent Gotchas

1. Do not use constructor-injected ITestOutputHelper in static methods. ITestOutputHelper is per-test-instance; store it in an instance field, not a static one.
2. Do not forget to make fixture classes public. xUnit requires fixture types to be public with a public parameterless constructor (or IAsyncLifetime). Non-public fixtures cause silent failures.
3. Do not mix [Fact] and [Theory] on the same method. A method is either a fact or a theory, not both.
4. Do not return void from async test methods. Return Task or ValueTask. async void tests report false success because xUnit cannot observe the async completion.
5. Do not use [Collection] without a matching [CollectionDefinition]. An unmatched collection name silently creates an implicit collection with default behavior, defeating the purpose.

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References

• xUnit Documentation
• xUnit v3 migration guide
• xUnit analyzers
• Shared context in xUnit
• Configuring xUnit with JSON