xUnit Fundamentals — Facts, Theories and Test Organisation

xUnit’s design philosophy is opinionated — it enforces isolation by creating a new class instance per test and provides minimal setup/teardown options. This pushes test writers toward clean, isolated tests rather than shared mutable state. Understanding xUnit’s key features — Facts, Theories, Fixtures, and parallelisation — is essential for writing fast, reliable test suites for the BlogApp’s C# code.

xUnit Core Features #

// ── [Theory] with multiple data sources ──────────────────────────────────
public class SlugGeneratorTests
{
    private readonly SlugGenerator _sut = new();

    // [InlineData] — simple inline values
    [Theory]
    [InlineData("Getting Started with .NET", "getting-started-with-net")]
    [InlineData("C# vs F# vs VB",            "c-vs-f-vs-vb")]
    [InlineData("  Spaces Around  ",          "spaces-around")]
    [InlineData("Hello World!",               "hello-world")]
    [InlineData("",                           "")]    // edge case
    public void Generate_ReturnsExpectedSlug(string title, string expected)
    {
        var result = _sut.Generate(title);
        result.Should().Be(expected);
    }

    // [MemberData] — complex test data from a static property
    public static IEnumerable<object[]> UnicodeTestCases =>
        new List<object[]>
        {
            new object[] { "Über alles",  "uber-alles"  },
            new object[] { "日本語",       ""             },  // non-Latin stripped
            new object[] { "café au lait", "cafe-au-lait" },
        };

    [Theory]
    [MemberData(nameof(UnicodeTestCases))]
    public void Generate_WithUnicodeInput_NormalisesCorrectly(
        string title, string expected)
    {
        _sut.Generate(title).Should().Be(expected);
    }
}

// ── IClassFixture — expensive shared setup per test class ─────────────────
// Database setup that should run once per class, not per test:
public class PostsRepositoryTests : IClassFixture<DatabaseFixture>
{
    private readonly DatabaseFixture _db;

    public PostsRepositoryTests(DatabaseFixture db) => _db = db;

    [Fact]
    public async Task GetBySlugAsync_WhenSlugExists_ReturnsPost()
    {
        var post = await _db.Repository.GetBySlugAsync("existing-slug", default);
        post.Should().NotBeNull();
        post!.Slug.Should().Be("existing-slug");
    }
}

// ── ICollectionFixture — share one fixture across multiple test classes ────
[CollectionDefinition("Database")]
public class DatabaseCollection : ICollectionFixture<DatabaseFixture> { }

[Collection("Database")]
public class CommentsRepositoryTests
{
    private readonly DatabaseFixture _db;
    public CommentsRepositoryTests(DatabaseFixture db) => _db = db;
    // ...
}

// ── ITestOutputHelper — diagnostic output in test results ─────────────────
public class PerformanceTests
{
    private readonly ITestOutputHelper _output;
    public PerformanceTests(ITestOutputHelper output) => _output = output;

    [Fact]
    public void GetPublished_ShouldCompleteUnder100ms()
    {
        var sw = Stopwatch.StartNew();
        // ... test code ...
        sw.Stop();
        _output.WriteLine($"Elapsed: {sw.ElapsedMilliseconds}ms");
        sw.ElapsedMilliseconds.Should().BeLessThan(100);
    }
}

// ── [Trait] — categorise tests for selective running ──────────────────────
[Trait("Category", "Unit")]
public class PostsServiceTests { /* ... */ }

[Trait("Category", "Integration")]
public class PostsControllerIntegrationTests { /* ... */ }

// Run only unit tests:
// dotnet test --filter "Category=Unit"

Common Mistakes #

Mistake 1 — Static shared state between xUnit test classes (test interference) #

❌ Wrong — static field holding test data modified by one test class; second parallel test class reads stale/modified data.

✅ Correct — no shared mutable static state; use IClassFixture for expensive setup; create fresh instances per test method.

Mistake 2 — [InlineData] with magic values (unclear test intent) #

❌ Wrong — [InlineData("a", true)]; what does “a” represent? Why should it be true?

✅ Correct — [InlineData("valid-slug", true)]; the value communicates its intent directly.