testing

Testing (.NET 10)

Core Principles

1. Integration tests are the highest-value tests — A single WebApplicationFactory test covers routing, binding, validation, business logic, and persistence in one shot. Start here before writing unit tests.
2. Real databases in tests — Use Testcontainers to spin up real PostgreSQL/SQL Server instances. In-memory providers hide real bugs (transactions, constraints, SQL generation).
3. AAA pattern is mandatory — Every test has three clearly separated sections: Arrange, Act, Assert. No mixing.
4. Test behavior, not implementation — Tests should survive refactoring. Test what the system does, not how it does it.

Patterns

xUnit v3 Basics

public class OrderServiceTests
{
    [Fact]
    public async Task CreateOrder_WithValidItems_ReturnsSuccessResult()
    {
        // Arrange
        var db = CreateInMemoryDb();
        var clock = new FakeTimeProvider(new DateTimeOffset(2025, 1, 15, 0, 0, 0, TimeSpan.Zero));
        var service = new OrderService(db, clock);
        var request = new CreateOrderRequest("customer-1", [new("product-1", 2)]);

        // Act
        var result = await service.CreateAsync(request);

        // Assert
        Assert.True(result.IsSuccess);
        Assert.NotEqual(Guid.Empty, result.Value.Id);
        Assert.Equal(clock.GetUtcNow(), result.Value.CreatedAt);
    }

    [Theory]
    [InlineData("")]
    [InlineData(null)]
    public async Task CreateOrder_WithInvalidCustomerId_ReturnsFailure(string? customerId)
    {
        // Arrange
        var service = CreateService();

        // Act
        var result = await service.CreateAsync(new CreateOrderRequest(customerId!, []));

        // Assert
        Assert.False(result.IsSuccess);
    }
}

Integration Tests with WebApplicationFactory

The highest-value test pattern. Tests the full HTTP pipeline.

// Fixtures/ApiFixture.cs
public class ApiFixture : WebApplicationFactory<Program>, IAsyncLifetime
{
    private readonly PostgreSqlContainer _postgres = new PostgreSqlBuilder()
        .WithImage("postgres:18")
        .Build();

    protected override void ConfigureWebHost(IWebHostBuilder builder)
    {
        builder.ConfigureServices(services =>
        {
            // Replace the real DB with Testcontainers
            services.RemoveAll<DbContextOptions<AppDbContext>>();
            services.AddDbContext<AppDbContext>(options =>
                options.UseNpgsql(_postgres.GetConnectionString()));
        });
    }

    // xUnit v3: IAsyncLifetime.InitializeAsync returns ValueTask (v2 used Task)
    public async ValueTask InitializeAsync()
    {
        await _postgres.StartAsync();

        // Apply migrations
        using var scope = Services.CreateScope();
        var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
        await db.Database.MigrateAsync();
    }

    // xUnit v3: IAsyncLifetime inherits IAsyncDisposable — override the
    // ValueTask DisposeAsync that WebApplicationFactory already provides
    public override async ValueTask DisposeAsync()
    {
        await _postgres.DisposeAsync();
        await base.DisposeAsync();
    }
}