tunit

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name: tunit description: Use this skill to write, review, and fix TUnit tests in .NET projects: for new test classes, assertions, data-driven tests, lifecycle hooks, debugging, migrating from xUnit/NUnit, choosing assertions, using Bogus, NSubstitute mocks, integration tests, or questions about parallelism and test ordering; prefer this skill over guessing, as TUnit's async-first API has non-obvious patterns differing from xUnit/NUnit.

TUnit Testing Skill

TUnit is a modern .NET testing framework that is async-first, source-generated, and runs on Microsoft.Testing.Platform.

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Quick-start anatomy

public class OrderHandlerTests
{
    [Test]
    [Category("Unit")]                             // categorise for filtering
    public async Task PlaceOrder_Valid_ReturnsOk()
    {
        // Arrange …
        // Act …
        // Assert — always await the assertion
        await Assert.That(result).IsNotNull();
        await Assert.That(result.Status).IsEqualTo("Confirmed");
    }
}

Key rule: every assertion line must be await-ed. Forgetting the await silently skips the assertion.

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Reference files — read before writing code

Topic	File
Assertion API (equality, nulls, booleans, collections, strings, exceptions, multiple)	references/assertions.md
All attributes (Test, Category, Arguments, Before/After, Retry, Repeat, NotInParallel…)	references/attributes.md
Data-driven tests (Arguments, MethodDataSource, ClassDataSource)	references/data-driven.md
Integration test patterns (Aspire, test infrastructure, async helpers)	references/integration-patterns.md

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Core rules

✅ [Test] async Task               ❌ [Fact] / [TestMethod] / [TestCase]
✅ await Assert.That(...)          ❌ Assert.Equal / FluentAssertions
✅ Per-test data creation          ❌ shared mutable state between tests

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Running tests

dotnet test                                              # all projects (parallel)
dotnet test -- --maximum-parallel-tests 4               # cap parallelism
dotnet test -- --treenode-filter "/*/*/*/*[Category=Unit]" # filter by category
dotnet test --project tests/MyProject.Tests/MyProject.Tests.csproj

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Common mistakes & fixes

Mistake	Fix
Assertion silently skipped	Add await to every Assert.That(…) call
Tests interfere with each other	Create all test data inside each test; never share mutable fields
Polling for eventual consistency	Use condition helpers or WaitForConditionAsync patterns; never Task.Delay
Assert.Fail not reached after exception	Use Assert.That(…).Throws<T>() pattern instead of try/catch
Missing [Before(Class)] / [After(Class)]	Hooks must be public static async Task; see attributes reference
Data-driven test parameters don't compile	Types in [Arguments] must exactly match method parameter types