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:17")
        .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()));
        });
    }

    public async Task InitializeAsync()
    {
        await _postgres.StartAsync();

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

    public new async Task DisposeAsync()
    {
        await _postgres.DisposeAsync();
        await base.DisposeAsync();
    }
}

// Tests/Orders/CreateOrderTests.cs
public class CreateOrderTests(ApiFixture fixture) : IClassFixture<ApiFixture>
{
    private readonly HttpClient _client = fixture.CreateClient();

    [Fact]
    public async Task CreateOrder_ReturnsCreated_WithValidRequest()
    {
        // Arrange
        var request = new CreateOrderRequest("customer-1", [new("product-1", 2)]);

        // Act
        var response = await _client.PostAsJsonAsync("/api/orders", request);

        // Assert
        Assert.Equal(HttpStatusCode.Created, response.StatusCode);

        var order = await response.Content.ReadFromJsonAsync<OrderResponse>();
        Assert.NotNull(order);
        Assert.NotEqual(Guid.Empty, order.Id);
        Assert.Contains("/api/orders/", response.Headers.Location?.ToString());
    }

    [Fact]
    public async Task CreateOrder_ReturnsValidationProblem_WithEmptyItems()
    {
        // Arrange
        var request = new CreateOrderRequest("customer-1", []);

        // Act
        var response = await _client.PostAsJsonAsync("/api/orders", request);

        // Assert
        Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
    }
}

Testcontainers for Real Database Testing

// For SQL Server
private readonly MsSqlContainer _mssql = new MsSqlBuilder()
    .WithImage("mcr.microsoft.com/mssql/server:2022-latest")
    .Build();

// For PostgreSQL
private readonly PostgreSqlContainer _postgres = new PostgreSqlBuilder()
    .WithImage("postgres:17")
    .Build();

// For Redis
private readonly RedisContainer _redis = new RedisBuilder()
    .WithImage("redis:7")
    .Build();

Verify Snapshot Testing

Use Verify for complex response objects where manual assertions would be fragile.

[Fact]
public async Task GetOrder_MatchesSnapshot()
{
    // Arrange
    await SeedOrder(fixture);

    // Act
    var response = await _client.GetAsync("/api/orders/known-id");
    var content = await response.Content.ReadAsStringAsync();

    // Assert — compares against a stored .verified.txt file
    await Verify(content);
}

On first run, Verify creates a .verified.txt file. On subsequent runs, it compares output. If the output changes, the test fails and shows a diff.

Test Data Builders

public class OrderBuilder
{
    private string _customerId = "default-customer";
    private List<OrderItem> _items = [new("product-1", 1, 9.99m)];
    private OrderStatus _status = OrderStatus.Pending;

    public OrderBuilder WithCustomer(string customerId)
    {
        _customerId = customerId;
        return this;
    }

    public OrderBuilder WithItems(params OrderItem[] items)
    {
        _items = [..items];
        return this;
    }

    public OrderBuilder WithStatus(OrderStatus status)
    {
        _status = status;
        return this;
    }

    public Order Build() => Order.Create(_customerId, _items, _status);
}

// Usage in tests
var order = new OrderBuilder()
    .WithCustomer("vip-customer")
    .WithStatus(OrderStatus.Confirmed)
    .Build();

Testing Time-Dependent Code

Use TimeProvider (built into .NET 8+) and FakeTimeProvider from Microsoft.Extensions.TimeProvider.Testing.

[Fact]
public async Task ExpireOrders_MarksOldPendingOrdersAsExpired()
{
    // Arrange
    var clock = new FakeTimeProvider(new DateTimeOffset(2025, 6, 1, 0, 0, 0, TimeSpan.Zero));
    var db = CreateDb();
    var order = Order.Create("customer-1", items, clock.GetUtcNow());
    db.Orders.Add(order);
    await db.SaveChangesAsync();

    // Advance time past expiry threshold
    clock.Advance(TimeSpan.FromDays(31));

    var handler = new ExpireOrders.Handler(db, clock);

    // Act
    await handler.Handle(new ExpireOrders.Command(), CancellationToken.None);

    // Assert
    var updated = await db.Orders.FindAsync(order.Id);
    Assert.Equal(OrderStatus.Expired, updated!.Status);
}

Test Naming Convention

Use the pattern: MethodName_StateUnderTest_ExpectedBehavior

[Fact] public async Task CreateOrder_WithValidItems_ReturnsSuccessResult() { }
[Fact] public async Task CreateOrder_WithEmptyItems_ReturnsValidationError() { }
[Fact] public async Task GetOrder_WithNonExistentId_ReturnsNotFound() { }
[Fact] public async Task CancelOrder_WhenAlreadyShipped_ReturnsConflict() { }

Anti-patterns

Don't Use In-Memory Database for Integration Tests

// BAD — hides real SQL behavior, transactions, constraints
services.AddDbContext<AppDbContext>(options =>
    options.UseInMemoryDatabase("TestDb"));

// GOOD — Testcontainers with real database
services.AddDbContext<AppDbContext>(options =>
    options.UseNpgsql(testContainer.GetConnectionString()));

Don't Test Implementation Details

// BAD — testing that a specific repository method was called
mock.Verify(x => x.AddAsync(It.IsAny<Order>()), Times.Once);
mock.Verify(x => x.SaveChangesAsync(), Times.Once);

// GOOD — test the observable outcome
var order = await db.Orders.FindAsync(orderId);
Assert.NotNull(order);
Assert.Equal(OrderStatus.Created, order.Status);

Don't Share Mutable State Between Tests

// BAD — static shared state
private static readonly AppDbContext SharedDb = CreateDb();

// GOOD — fresh state per test (or use IAsyncLifetime for shared fixtures)
private AppDbContext CreateDb() => new(new DbContextOptionsBuilder<AppDbContext>()...);

Don't Write Assertion-Free Tests

// BAD — no assertion, only checks it doesn't throw
[Fact]
public async Task CreateOrder_Works()
{
    await service.CreateAsync(request);
    // "it didn't throw, so it works!" — NO
}

// GOOD — assert the expected outcome
[Fact]
public async Task CreateOrder_PersistsOrderToDatabase()
{
    var result = await service.CreateAsync(request);

    var persisted = await db.Orders.FindAsync(result.Value.Id);
    Assert.NotNull(persisted);
    Assert.Equal(request.CustomerId, persisted.CustomerId);
}

Decision Guide

Scenario	Recommendation
Testing an API endpoint	WebApplicationFactory integration test
Testing business logic in isolation	Unit test with fakes/stubs
Database-dependent tests	Testcontainers (real DB)
Complex response validation	Verify snapshot testing
Time-dependent logic	FakeTimeProvider
External API dependency	WireMock.Net or HttpMessageHandler stub
Parameterized test cases	[Theory] with [InlineData] or [MemberData]
Test data setup	Builder pattern
Shared expensive fixture	IClassFixture<T> with IAsyncLifetime