xUnit - Test Structure & Lifecycle

using Xunit;
using FluentAssertions;

// ━━ [Fact] - single test case ━━
public class SlugGeneratorTests
{
    private readonly SlugGenerator _sut; // sut = System Under Test

    public SlugGeneratorTests() =>
        _sut = new SlugGenerator(); // fresh instance per test - xUnit's design

    [Fact]
    public void GenerateSlug_WithSimpleText_ReturnsLowercase()
    {
        // Arrange
        var input = "Hello World";

        // Act
        var slug = _sut.Generate(input);

        // Assert - FluentAssertions: readable, produces great error messages
        slug.Should().Be("hello-world");
    }

    [Fact]
    public void GenerateSlug_WithSpecialChars_RemovesNonAlphanumeric()
    {
        var slug = _sut.Generate("C# & .NET Development");
        slug.Should().Be("c-net-development");
        slug.Should().NotContain("#").And.NotContain("&").And.NotContain(" ");
    }

    // ━━ [Theory] + [InlineData] - parametric tests ━━
    [Theory]
    [InlineData("Hello World",      "hello-world")]
    [InlineData("C# Programming",   "c-programming")]
    [InlineData("  leading spaces", "leading-spaces")]
    [InlineData("multiple   spaces","multiple-spaces")]
    [InlineData("UPPER CASE",       "upper-case")]
    public void GenerateSlug_VariousInputs_ReturnsExpectedSlug(string input, string expected)
    {
        _sut.Generate(input).Should().Be(expected);
    }

    // ━━ [Theory] + [MemberData] - complex data from static method ━━
    public static IEnumerable<object[]> SlugEdgeCases() =>
    [
        [string.Empty,     ""],
        ["   ",            ""],
        ["a",              "a"],
        [new string('a', 300), new string('a', 250)], // truncation at 250 chars
    ];

    [Theory, MemberData(nameof(SlugEdgeCases))]
    public void GenerateSlug_EdgeCases(string input, string expected) =>
        _sut.Generate(input).Should().Be(expected);
}

// ━━ Test fixtures - shared expensive setup (database, server) ━━
// IAsyncLifetime: async setup/teardown per test class
public class ProductServiceTests : IAsyncLifetime
{
    private AppDbContext _db = null!;

    public async Task InitializeAsync()
    {
        // Called once before any test in this class
        _db = await TestDbContextFactory.CreateInMemoryAsync();
        await _db.Products.AddRangeAsync(TestData.SampleProducts());
        await _db.SaveChangesAsync();
    }

    public async Task DisposeAsync()
    {
        await _db.DisposeAsync();
    }

    [Fact]
    public async Task GetByIdAsync_ExistingProduct_ReturnsProduct()
    {
        // Arrange
        var existingId = TestData.SampleProducts().First().Id;

        // Act
        var product = await _db.Products.FindAsync(existingId);

        // Assert
        product.Should().NotBeNull();
        product!.Id.Should().Be(existingId);
    }
}

// Stub types for compilation
class SlugGenerator { public string Generate(string input) => input.ToLower().Replace(" ", "-").Replace("#", "").Replace("&", "").Trim(); }
class AppDbContext { public System.Collections.Generic.List<Product> Products { get; set; } = []; public Task<int> SaveChangesAsync() => Task.FromResult(0); public void Dispose() { } public ValueTask DisposeAsync() => ValueTask.CompletedTask; }
static class TestDbContextFactory { public static Task<AppDbContext> CreateInMemoryAsync() => Task.FromResult(new AppDbContext()); }
static class TestData { public static System.Collections.Generic.List<Product> SampleProducts() => [new Product { Id = Guid.NewGuid(), Name = "Test Product", Price = 9.99m }]; }
class Product { public Guid Id { get; set; } public string Name { get; set; } = ""; public decimal Price { get; set; } }