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Writing xUnit tests. v3 Fact/Theory, fixtures, parallelism, IAsyncLifetime, v2 compatibility.
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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.
| 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.
[Fact] -- Single Test CaseUse [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 TestsUse [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 provide shared, expensive resources across tests while maintaining test isolation.
IClassFixture<T> -- Shared Per Test ClassUse 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 ClassesUse 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 ClassesFor 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);
}
}
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.
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
}
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.
ITestOutputHelperCapture 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($"Processed {result.Count} items in {sw.ElapsedMilliseconds}ms");
Assert.True(sw.Elapsed < TimeSpan.FromSeconds(5));
}
}
ILoggerBridge xUnit output to Microsoft.Extensions.Logging for integration tests:
// NuGet: Microsoft.Extensions.Logging (for LoggerFactory)
// + a logging provider that writes to ITestOutputHelper
// Common approach: use a simple adapter
public class XunitLoggerProvider : ILoggerProvider
{
private readonly ITestOutputHelper _output;
public XunitLoggerProvider(ITestOutputHelper output) => _output = output;
public ILogger CreateLogger(string categoryName) =>
new XunitLogger(_output, categoryName);
public void Dispose() { }
}
public class XunitLogger(ITestOutputHelper output, string category) : ILogger
{
public IDisposable? BeginScope<TState>(TState state) where TState : notnull => null;
public bool IsEnabled(LogLevel logLevel) => true;
public void Log<TState>(LogLevel logLevel, EventId eventId, TState state,
Exception? exception, Func<TState, Exception?, string> formatter)
{
output.WriteLine($"[{logLevel}] {category}: {formatter(state, exception)}");
if (exception is not null)
output.WriteLine(exception.ToString());
}
}
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);
}
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.
The xunit.analyzers package (included with xUnit v3) catches common test authoring mistakes at compile time.
| 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 |
In .editorconfig for test projects:
[tests/**.cs]
# Allow skipped tests during development
dotnet_diagnostic.xUnit1004.severity = suggestion
[Fact], one concept per [Theory]. If a [Theory] tests fundamentally different scenarios, split into separate [Fact] methods.IClassFixture for expensive shared resources within a single test class. Use ICollectionFixture when multiple classes share the same resource.IAsyncLifetime for async setup/teardown instead of constructors and IDisposable. Constructors cannot be async, and IDisposable.Dispose() does not await.[InlineData] for simple cases. Use [MemberData] or [ClassData] only when data is complex or shared.ITestOutputHelper in static methods. ITestOutputHelper is per-test-instance; store it in an instance field, not a static one.public. xUnit requires fixture types to be public with a public parameterless constructor (or IAsyncLifetime). Non-public fixtures cause silent failures.[Fact] and [Theory] on the same method. A method is either a fact or a theory, not both.void from async test methods. Return Task or ValueTask. async void tests report false success because xUnit cannot observe the async completion.[Collection] without a matching [CollectionDefinition]. An unmatched collection name silently creates an implicit collection with default behavior, defeating the purpose.