dotnet-integration-testing

What this skill does

# dotnet-integration-testing

Integration testing patterns for .NET applications using WebApplicationFactory, Testcontainers, and .NET Aspire testing. Covers in-process API testing, disposable infrastructure via containers, database fixture management, and test isolation strategies.

**Version assumptions:** .NET 8.0+ baseline, Testcontainers 3.x+, .NET Aspire 9.0+. Package versions for `Microsoft.AspNetCore.Mvc.Testing` must match the project's target framework major version (e.g., 8.x for net8.0, 9.x for net9.0, 10.x for net10.0). Examples below use Testcontainers 4.x APIs; the patterns apply equally to 3.x with minor namespace differences.

**Out of scope:** Test project scaffolding (creating projects, package references) is owned by [skill:dotnet-add-testing]. Testing strategy and test type selection are covered by [skill:dotnet-testing-strategy]. Snapshot testing for verifying API response structures is covered by [skill:dotnet-snapshot-testing].

**Prerequisites:** Test project already scaffolded via [skill:dotnet-add-testing] with integration test packages referenced. Docker daemon running (required by Testcontainers). Run [skill:dotnet-version-detection] to confirm .NET 8.0+ baseline.

Cross-references: [skill:dotnet-testing-strategy] for deciding when integration tests are appropriate, [skill:dotnet-xunit] for xUnit fixtures and parallel execution configuration, [skill:dotnet-snapshot-testing] for verifying API response structures with Verify.

---

## WebApplicationFactory

`WebApplicationFactory<TEntryPoint>` creates an in-process test server for ASP.NET Core applications. Tests send HTTP requests without network overhead, exercising the full middleware pipeline, routing, model binding, and serialization.

### Package

```xml
<!-- Version must match target framework: 8.x for net8.0, 9.x for net9.0, etc. -->
<PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" />
```

### Basic Usage

```csharp
public class OrdersApiTests : IClassFixture<WebApplicationFactory<Program>>
{
    private readonly HttpClient _client;

    public OrdersApiTests(WebApplicationFactory<Program> factory)
    {
        _client = factory.CreateClient();
    }

    [Fact]
    public async Task GetOrders_ReturnsOkWithJsonArray()
    {
        var response = await _client.GetAsync("/api/orders");

        response.EnsureSuccessStatusCode();
        var orders = await response.Content
            .ReadFromJsonAsync<List<OrderDto>>();
        Assert.NotNull(orders);
    }

    [Fact]
    public async Task CreateOrder_ValidPayload_Returns201()
    {
        var request = new CreateOrderRequest
        {
            CustomerId = "cust-123",
            Items = [new("SKU-001", Quantity: 2)]
        };

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

        Assert.Equal(HttpStatusCode.Created, response.StatusCode);
        Assert.NotNull(response.Headers.Location);
    }
}
```

**Important:** The `Program` class must be accessible to the test project. Either make it public or add an `InternalsVisibleTo` attribute:

```csharp
// In the API project (e.g., Program.cs or a separate file)
[assembly: InternalsVisibleTo("MyApp.Api.IntegrationTests")]
```

Or in the csproj:

```xml
<ItemGroup>
  <InternalsVisibleTo Include="MyApp.Api.IntegrationTests" />
</ItemGroup>
```

### Customizing the Test Server

Override services, configuration, or middleware using `WebApplicationFactory<T>.WithWebHostBuilder`:

```csharp
public class CustomWebAppFactory : WebApplicationFactory<Program>
{
    // Provide connection string from test fixture (e.g., Testcontainers)
    public string ConnectionString { get; set; } = "";

    protected override void ConfigureWebHost(IWebHostBuilder builder)
    {
        builder.UseEnvironment("Testing");

        builder.ConfigureAppConfiguration((context, config) =>
        {
            config.AddInMemoryCollection(new Dictionary<string, string?>
            {
                ["ConnectionStrings:Default"] = ConnectionString,
                ["Features:EnableNewCheckout"] = "true"
            });
        });

        builder.ConfigureTestServices(services =>
        {
            // Replace real services with test doubles
            services.RemoveAll<IEmailSender>();
            services.AddSingleton<IEmailSender, FakeEmailSender>();

            // Replace database context with test database
            services.RemoveAll<DbContextOptions<AppDbContext>>();
            services.AddDbContext<AppDbContext>(options =>
                options.UseNpgsql(ConnectionString));
        });
    }
}
```

### Authenticated Requests

Test authenticated endpoints by configuring an authentication handler:

```csharp
public class AuthenticatedWebAppFactory : WebApplicationFactory<Program>
{
    protected override void ConfigureWebHost(IWebHostBuilder builder)
    {
        builder.ConfigureTestServices(services =>
        {
            services.AddAuthentication("Test")
                .AddScheme<AuthenticationSchemeOptions, TestAuthHandler>(
                    "Test", options => { });
        });
    }
}

public class TestAuthHandler : AuthenticationHandler<AuthenticationSchemeOptions>
{
    public TestAuthHandler(
        IOptionsMonitor<AuthenticationSchemeOptions> options,
        ILoggerFactory logger,
        UrlEncoder encoder)
        : base(options, logger, encoder) { }

    protected override Task<AuthenticateResult> HandleAuthenticateAsync()
    {
        var claims = new[]
        {
            new Claim(ClaimTypes.NameIdentifier, "test-user-id"),
            new Claim(ClaimTypes.Name, "Test User"),
            new Claim(ClaimTypes.Role, "Admin")
        };
        var identity = new ClaimsIdentity(claims, "Test");
        var principal = new ClaimsPrincipal(identity);
        var ticket = new AuthenticationTicket(principal, "Test");

        return Task.FromResult(AuthenticateResult.Success(ticket));
    }
}
```

---

## Testcontainers

Testcontainers spins up real infrastructure (databases, message brokers, caches) in Docker containers for tests. Each test run gets a fresh, disposable environment.

### Packages

```xml
<PackageReference Include="Testcontainers" Version="4.*" />
<!-- Database-specific modules -->
<PackageReference Include="Testcontainers.PostgreSql" Version="4.*" />
<PackageReference Include="Testcontainers.MsSql" Version="4.*" />
<PackageReference Include="Testcontainers.Redis" Version="4.*" />
```

### PostgreSQL Example

```csharp
public class PostgresFixture : IAsyncLifetime
{
    private readonly PostgreSqlContainer _container = new PostgreSqlBuilder()
        .WithImage("postgres:16-alpine")
        .WithDatabase("testdb")
        .WithUsername("test")
        .WithPassword("test")
        .Build();

    public string ConnectionString => _container.GetConnectionString();

    public async ValueTask InitializeAsync()
    {
        await _container.StartAsync();
    }

    public async ValueTask DisposeAsync()
    {
        await _container.DisposeAsync();
    }
}

[CollectionDefinition("Postgres")]
public class PostgresCollection : ICollectionFixture<PostgresFixture> { }

[Collection("Postgres")]
public class OrderRepositoryTests
{
    private readonly PostgresFixture _postgres;

    public OrderRepositoryTests(PostgresFixture postgres)
    {
        _postgres = postgres;
    }

    [Fact]
    public async Task Insert_ValidOrder_CanBeRetrieved()
    {
        await using var context = CreateContext(_postgres.ConnectionString);
        await context.Database.EnsureCreatedAsync();

        var order = new Order { CustomerId = "cust-1", Total = 99.99m };
        context.Orders.Add(order);
        await context.SaveChangesAsync();

        var retrieved = await context.Orders.FindAsync(order.Id);
        Assert.NotNull(retrieved);
        Assert.Equal(99.99m, retrieved.Total);
    }

    private static AppDbContext CreateContext(string connectionString)
    {
        var options = new DbContextOptionsBuild