testing

---
name: testing
description: >
license: MIT
---

# 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

```csharp
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.

```csharp
// 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();
    }
}
```

```csharp
// 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

```csharp
// 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:18")
    .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.

```csharp
[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

```csharp
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`.

```csharp
[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`

```csharp
[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

```csharp
// 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

```csharp
// 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

```csharp
// 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

```csharp
// 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` |