Testing with TUnit

When to Use This Skill

Use this skill when:

• Creating a new TUnit test project or adding TUnit to an existing solution
• Writing unit, integration, or acceptance tests using TUnit
• Migrating tests from xUnit, NUnit, or MSTest to TUnit
• Configuring data-driven tests with [Arguments], [MethodDataSource], or [ClassDataSource]
• Setting up test lifecycle hooks ([Before]/[After])
• Controlling parallelism with [NotInParallel], [DependsOn], or parallel groups
• Writing ASP.NET Core integration tests with TUnit.AspNetCore
• Configuring TUnit for CI/CD pipelines with coverage and TRX reports

---

What is TUnit?

TUnit is a modern, source-generated testing framework for .NET built on the Microsoft Testing Platform. Key characteristics:

• Source generated - Tests are discovered at compile time, not via reflection
• Parallel by default - Tests run concurrently for speed
• Async-first assertions - All assertions must be awaited
• New class instance per test - Test classes are instantiated fresh for each test method
• No [TestClass] attribute needed - Only [Test] on methods
• Native AOT and single-file support - Works where reflection-based frameworks cannot
• Built-in code coverage and TRX reports - No need for Coverlet

---

Installation

From Template (Recommended)

dotnet new install TUnit.Templates
dotnet new TUnit -n "MyApp.Tests"

Manual Setup

dotnet new console --name MyApp.Tests
cd MyApp.Tests
dotnet add package TUnit --prerelease

Remove any auto-generated Program.cs -- TUnit handles the entry point.

Project File

<Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net9.0</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
    </PropertyGroup>
    <ItemGroup>
        <PackageReference Include="TUnit" Version="*" />
    </ItemGroup>
</Project>

CRITICAL: Do NOT Use These Packages

Package	Why
Microsoft.NET.Test.Sdk	Breaks TUnit test discovery -- TUnit uses Microsoft.Testing.Platform, not VSTest
coverlet.collector / coverlet.msbuild	Incompatible with TUnit -- use the built-in --coverage flag instead

Global Usings

TUnit automatically provides global usings for TUnit.Core, TUnit.Assertions, and TUnit.Assertions.Extensions. You do not need explicit using statements in test files.

---

Writing Tests

Basic Test

namespace MyApp.Tests;

public class CalculatorTests
{
    [Test]
    public async Task Add_TwoNumbers_ReturnsSum()
    {
        var result = 2 + 3;

        await Assert.That(result).IsEqualTo(5);
    }
}

Test Method Signatures

[Test]
public void SyncTest()           // Valid -- synchronous, no assertions
{
    var result = Calculate(2, 3);
}

[Test]
public async Task AsyncTest()    // Recommended -- required if using assertions
{
    await Assert.That(42).IsEqualTo(42);
}

// async void is NOT allowed -- compiler error

Rule: If you use Assert.That(...), the test method must be async Task because assertions are awaitable.

---

Assertions

All TUnit assertions follow the pattern await Assert.That(actual).SomeCondition().

Core Assertions

// Equality
await Assert.That(result).IsEqualTo(5);
await Assert.That(result).IsNotEqualTo(0);

// Comparison
await Assert.That(score).IsGreaterThan(70);
await Assert.That(age).IsLessThanOrEqualTo(100);
await Assert.That(temp).IsBetween(20, 30);

// Boolean
await Assert.That(isValid).IsTrue();
await Assert.That(isDeleted).IsFalse();

// Null
await Assert.That(result).IsNotNull();
await Assert.That(optional).IsNull();

// Type
await Assert.That(obj).IsTypeOf<MyClass>();

String Assertions

await Assert.That(message).Contains("Hello");
await Assert.That(filename).StartsWith("test_");
await Assert.That(email).Matches(@"^[\w\.-]+@[\w\.-]+\.\w+$");
await Assert.That(input).IsNotEmpty();

Collection Assertions

await Assert.That(numbers).Contains(42);
await Assert.That(items).Count().IsEqualTo(5);
await Assert.That(list).IsNotEmpty();
await Assert.That(values).All(x => x > 0);
await Assert.That(numbers).IsEquivalentTo(new[] { 5, 4, 3, 2, 1 }); // order-independent
await Assert.That(numbers).IsInOrder();

Exception Assertions

// Basic exception testing
await Assert.That(() => int.Parse("not a number"))
    .Throws<FormatException>();

// Async exception testing
await Assert.That(async () => await FailingOperationAsync())
    .Throws<HttpRequestException>();

// Exact type (no subclasses)
await Assert.That(() => throw new ArgumentNullException())
    .ThrowsExactly<ArgumentNullException>();

// Exception message
await Assert.That(() => throw new InvalidOperationException("Operation failed"))
    .Throws<InvalidOperationException>()
    .WithMessage("Operation failed");

await Assert.That(() => throw new ArgumentException("The parameter 'userId' is invalid"))
    .Throws<ArgumentException>()
    .WithMessageContaining("userId");

// ArgumentException parameter name
await Assert.That(() => ValidateUser(null!))
    .Throws<ArgumentNullException>()
    .WithParameterName("user");

// Inner exceptions
await Assert.That(() => ThrowWithInner())
    .Throws<InvalidOperationException>()
    .WithInnerException()
    .Throws<FormatException>();

// No exception thrown
await Assert.That(() => int.Parse("42"))
    .ThrowsNothing();

Chaining with And / Or

await Assert.That(username)
    .IsNotNull()
    .And.IsNotEmpty()
    .And.Length().IsGreaterThan(3)
    .And.Length().IsLessThan(20);

await Assert.That(statusCode)
    .IsEqualTo(200)
    .Or.IsEqualTo(201)
    .Or.IsEqualTo(204);

Assert.Multiple (Report All Failures)

using (Assert.Multiple())
{
    await Assert.That(user.FirstName).IsEqualTo("John");
    await Assert.That(user.LastName).IsEqualTo("Doe");
    await Assert.That(user.Age).IsGreaterThan(18);
}
// All failures reported together, not just the first one

Floating-Point Tolerance

await Assert.That(3.14159).IsEqualTo(Math.PI).Within(0.001);

CRITICAL: Always Await Assertions

// WRONG -- assertion never executes, test always passes
Assert.That(result).IsEqualTo(5);

// CORRECT
await Assert.That(result).IsEqualTo(5);

TUnit includes a built-in analyzer that warns about unawaited assertions.

---

Data-Driven Tests

[Arguments] -- Compile-Time Constants

[Test]
[Arguments(1, 1, 2)]
[Arguments(1, 2, 3)]
[Arguments(2, 2, 4)]
public async Task Add_ReturnsExpectedResult(int a, int b, int expected)
{
    await Assert.That(a + b).IsEqualTo(expected);
}

Supports metadata: DisplayName, Categories, Skip:

[Test]
[Arguments("Chrome", "120")]
[Arguments("Safari", "17", Skip = "Safari not available in CI")]
public async Task BrowserTest(string browser, string version) { }

[MethodDataSource] -- Dynamic/Complex Data

public static class TestData
{
    public static IEnumerable<Func<(int A, int B, int Expected)>> AdditionCases()
    {
        yield return () => (1, 2, 3);
        yield return () => (2, 2, 4);
        yield return () => (5, 5, 10);
    }
}

public class MathTests
{
    [Test]
    [MethodDataSource(typeof(TestData), nameof(TestData.AdditionCases))]
    public async Task Add_WithData(int a, int b, int expected)
    {
        await Assert.That(a + b).IsEqualTo(expected);
    }
}

For reference types, return Func<T> (not T) to ensure each test gets a fresh instance.

[ClassDataSource] -- Injectable Shared Resources

public class TestWebServer : IAsyncInitializer, IAsyncDisposable
{
    public WebApplicationFactory<Program>? Factory { get; private set; }

    public async Task InitializeAsync()
    {
        Factory = new WebApplicationFactory<Program>();
        await Task.CompletedTask;
    }

    public async ValueTask DisposeAsync()
    {
        if (Factory != null) await Factory.DisposeAsync();
    }
}

[ClassDataSource<TestWebServer>(Shared = SharedType.PerTestSession)]
public class ApiTests(TestWebServer server)
{
    [Test]
    public async Task HealthCheck_ReturnsOk()
    {
        var client = server.Factory!.CreateClient();
        var response = await client.GetAsync("/health");

        await Assert.That(response.IsSuccessStatusCode).IsTrue();
    }
}

SharedType options:

• None (default) -- new instance per test
• PerClass -- shared within the test class
• PerAssembly -- shared within the assembly
• PerTestSession -- single instance for entire test run
• Keyed -- shared among tests with the same Key

---

Test Lifecycle

Instance Per Test

TUnit creates a new instance of the test class for each test method. Instance fields are never shared between tests.

public class MyTests
{
    private int _value;

    [Test, NotInParallel]
    public void Test1() { _value = 99; }

    [Test, NotInParallel]
    public async Task Test2()
    {
        // _value is 0 here -- different instance!
        await Assert.That(_value).IsEqualTo(0);
    }
}

Use static fields if you intentionally need shared state.

Setup Hooks -- [Before]

public class DatabaseTests
{
    private TestDatabase? _database;

    [Before(Test)]    // Instance method, runs before each test
    public async Task SetupDatabase()
    {
        _database = await TestDatabase.CreateAsync();
    }

    [Before(Class)]   // Must be static, runs once before all tests in class
    public static async Task ClassSetup()
    {
        await GlobalResource.InitializeAsync();
    }

    [Before(Assembly)] // Must be static, runs once before all tests in assembly
    public static async Task AssemblySetup() { }
}

Cleanup Hooks -- [After]

public class DatabaseTests
{
    [After(Test)]     // Instance method, runs after each test
    public async Task Cleanup()
    {
        if (_database != null) await _database.DisposeAsync();
    }

    [After(Class)]    // Must be static, runs once after all tests in class
    public static async Task ClassCleanup() { }
}

Every [After] method runs even if a previous one fails. Exceptions are aggregated.

Hook Levels

Level	Scope	Static?
[Before(Test)] / [After(Test)]	Each test	Instance
[Before(Class)] / [After(Class)]	Once per class	Static
[Before(Assembly)] / [After(Assembly)]	Once per assembly	Static
[Before(TestSession)] / [After(TestSession)]	Once per test run	Static

Global Hooks -- [BeforeEvery] / [AfterEvery]

Place in a GlobalHooks.cs at the project root:

public static class GlobalHooks
{
    [BeforeEvery(Test)]
    public static void BeforeEachTest(TestContext context)
    {
        Console.WriteLine($"Starting: {context.Metadata.TestName}");
    }

    [AfterEvery(Test)]
    public static async Task AfterEachTest(TestContext context)
    {
        if (context.Execution.Result?.State == TestState.Failed)
        {
            await CaptureScreenshotAsync();
        }
    }
}

Hook Parameters

Hooks can accept context and cancellation token:

[Before(Test)]
public async Task Setup(TestContext context, CancellationToken ct)
{
    Console.WriteLine($"Setting up: {context.Metadata.TestName}");
    await SomeOperation(ct);
}

---

Parallelism

Default: Tests Run in Parallel

TUnit runs all tests concurrently by default. Write independent, stateless tests.

[NotInParallel] -- Disable for Specific Tests

[Test, NotInParallel]
public async Task ModifiesSharedResource() { }

Constraint Keys -- Parallel Groups

// These two won't run in parallel with each other (shared key)
[Test, NotInParallel("DatabaseTest")]
public async Task DbTest1() { }

[Test, NotInParallel("DatabaseTest")]
public async Task DbTest2() { }

// This can still run in parallel with the above
[Test, NotInParallel("FileTest")]
public async Task FileTest1() { }

[DependsOn] -- Order with Parallelism

[Test]
public async Task Step1_CreateUser() { }

[Test]
[DependsOn(nameof(Step1_CreateUser))]
public async Task Step2_UpdateUser() { }

[Test]
[DependsOn(nameof(Step2_UpdateUser))]
public async Task Step3_DeleteUser() { }
// Other unrelated tests still run in parallel

Disable All Parallelism

[assembly: NotInParallel]

Limit Concurrent Tests (CLI)

dotnet run -c Release --maximum-parallel-tests 8

---

Dependency Injection

public class MicrosoftDiDataSourceAttribute
    : DependencyInjectionDataSourceAttribute<IServiceScope>
{
    private static readonly IServiceProvider ServiceProvider = CreateProvider();

    public override IServiceScope CreateScope(DataGeneratorMetadata metadata)
        => ServiceProvider.CreateScope();

    public override object? Create(IServiceScope scope, Type type)
        => scope.ServiceProvider.GetService(type);

    private static IServiceProvider CreateProvider()
        => new ServiceCollection()
            .AddSingleton<IMyService, MyService>()
            .AddTransient<IRepository, Repository>()
            .BuildServiceProvider();
}

[MicrosoftDiDataSource]
public class ServiceTests(IMyService service, IRepository repo)
{
    [Test]
    public async Task ServiceWorks()
    {
        var result = await service.DoWorkAsync();
        await Assert.That(result).IsNotNull();
    }
}

---

ASP.NET Core Integration Testing

Install the TUnit.AspNetCore package:

dotnet add package TUnit.AspNetCore

Factory + Base Class Pattern

using TUnit.AspNetCore;

public class AppFactory : TestWebApplicationFactory<Program>
{
    protected override void ConfigureWebHost(IWebHostBuilder builder)
    {
        builder.ConfigureAppConfiguration((_, config) =>
        {
            config.AddInMemoryCollection(new Dictionary<string, string?>
            {
                { "ConnectionStrings:Default", "..." }
            });
        });
    }
}

public abstract class IntegrationTestBase : WebApplicationTest<AppFactory, Program> { }

Writing Integration Tests

public class TodoApiTests : IntegrationTestBase
{
    [Test]
    public async Task GetTodos_ReturnsOk()
    {
        var client = Factory.CreateClient();
        var response = await client.GetAsync("/todos");

        await Assert.That(response.StatusCode).IsEqualTo(HttpStatusCode.OK);
    }

    // Override per-test services
    protected override void ConfigureTestServices(IServiceCollection services)
    {
        services.ReplaceService<IEmailService>(new FakeEmailService());
    }

    // Override per-test configuration
    protected override void ConfigureTestConfiguration(IConfigurationBuilder config)
    {
        config.AddInMemoryCollection(new Dictionary<string, string?>
        {
            { "Feature:Enabled", "true" }
        });
    }
}

Test Isolation with Shared Containers

Each test gets a unique ID for resource isolation:

public abstract class DatabaseTestBase : IntegrationTestBase
{
    protected string TableName { get; private set; } = null!;

    protected override async Task SetupAsync()
    {
        TableName = GetIsolatedName("todos"); // "Test_42_todos"
        await CreateTableAsync(TableName);
    }

    protected override void ConfigureTestConfiguration(IConfigurationBuilder config)
    {
        config.AddInMemoryCollection(new Dictionary<string, string?>
        {
            { "Database:TableName", TableName }
        });
    }

    [After(HookType.Test)]
    public async Task Cleanup() => await DropTableAsync(TableName);
}

---

Running Tests

Command Line

# Preferred -- easier flag passing
dotnet run -c Release

# With coverage and TRX report
dotnet run -c Release --coverage --report-trx

# Using dotnet test (flags go after --)
dotnet test -c Release -- --coverage --report-trx

IDE Support

IDE	Setting Required
Visual Studio	Enable "Use testing platform server mode" in Tools > Manage Preview Features
Rider	Enable "Testing Platform support" in Settings > Build, Execution, Deployment > Unit Testing > Testing Platform
VS Code	Install C# Dev Kit, enable "Dotnet > Test Window > Use Testing Platform Protocol"

---

CI/CD Integration

GitHub Actions

name: Tests
on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v4
    - uses: actions/setup-dotnet@v4
      with:
        dotnet-version: 9.0.x
    - run: dotnet restore
    - run: dotnet build --no-restore -c Release
    - run: dotnet run --project tests/MyApp.Tests -c Release --no-build -- --report-trx --coverage
    - uses: actions/upload-artifact@v4
      if: always()
      with:
        name: test-results
        path: |
          **/TestResults/*.trx
          **/TestResults/**/coverage.cobertura.xml

---

Test Naming Conventions

Use descriptive names: Method_Scenario_ExpectedBehavior

[Test]
public async Task CalculateTotal_WithDiscount_ReturnsReducedPrice() { }

[Test]
public async Task CreateUser_WithDuplicateEmail_ThrowsConflictException() { }

---

Test Organization

Mirror production code structure:

MyApp/
  Services/
    OrderService.cs
MyApp.Tests/
  Services/
    OrderServiceTests.cs

Common Mistakes

Forgetting to Await Assertions

// WRONG -- silently passes
Assert.That(result).IsEqualTo(5);

// CORRECT
await Assert.That(result).IsEqualTo(5);

Using Instance State Between Tests

// WRONG -- each test gets a new class instance
private int _counter;

[Test, NotInParallel]
public void Increment() { _counter++; }

[Test, NotInParallel]
public async Task Check()
{
    await Assert.That(_counter).IsEqualTo(1); // Fails -- _counter is 0
}

Using Microsoft.NET.Test.Sdk

This package is for VSTest-based frameworks. TUnit uses Microsoft.Testing.Platform. Including it will break test discovery.

Relying on Test Execution Order

Tests run in parallel by default. Never assume order unless you use [DependsOn] or [NotInParallel(Order = N)].

Over-Mocking

// BAD -- mock everything
var mockLogger = new Mock<ILogger>();
var mockValidator = new Mock<IValidator>();
var mockCalculator = new Mock<IPriceCalculator>();

// BETTER -- only mock external dependencies
var logger = NullLogger.Instance;
var validator = new OrderValidator();            // Real, fast
var mockRepository = new Mock<IOrderRepository>(); // Mock database

---

Best Practices Summary

Practice	Why
Always await assertions	Unawaited assertions silently pass
Use async Task for test methods	Required by TUnit's assertion model
One logical behavior per test	Keeps tests focused and failure messages clear
Use Assert.Multiple for related checks	See all failures at once
Prefer [DependsOn] over [NotInParallel(Order)]	Maintains parallelism for unrelated tests
Use [ClassDataSource] for expensive resources	Share across tests with SharedType.PerTestSession
Test behavior, not implementation	Avoid brittle mock-verification tests
Use GetIsolatedName() in integration tests	Ensures parallel test isolation
Place [BeforeEvery]/[AfterEvery] in GlobalHooks.cs	Easy to find global hooks
Do not install Microsoft.NET.Test.Sdk	Breaks TUnit test discovery

---

Resources

• TUnit Docs: https://tunit.dev/docs/intro
• GitHub: https://github.com/thomhurst/TUnit
• NuGet: https://www.nuget.org/packages/TUnit
• TUnit.AspNetCore: https://www.nuget.org/packages/TUnit.AspNetCore
• Migration from xUnit: https://tunit.dev/docs/migration/xunit
• Migration from NUnit: https://tunit.dev/docs/migration/nunit
• Migration from MSTest: https://tunit.dev/docs/migration/mstest