WebApplicationFactory — In-Memory API Testing

WebApplicationFactory<TEntryPoint> spins up the complete ASP.NET Core application in memory — the same routing, middleware, DI container, and controllers as production, but with a test HTTP client instead of a real socket. Integration tests using WebApplicationFactory test the entire vertical slice: from HTTP request parsing through controller action to database (or test substitute) and back. They are slower than unit tests but catch bugs that unit tests miss — middleware ordering, model binding, validation pipeline, response serialisation.

BlogAppFactory Setup #

// ── Custom WebApplicationFactory ──────────────────────────────────────────
public class BlogAppFactory : WebApplicationFactory<Program>, IAsyncLifetime
{
    // Unique DB name per factory instance to enable parallel test classes
    private readonly string _dbName = $"BlogApp_Test_{Guid.NewGuid():N}";

    protected override void ConfigureWebHost(IWebHostBuilder builder)
    {
        builder.ConfigureTestServices(services =>
        {
            // ── Replace production DbContext with test database ────────────
            // Remove production SQL Server registration
            var descriptor = services.SingleOrDefault(d =>
                d.ServiceType == typeof(DbContextOptions<AppDbContext>));
            if (descriptor != null) services.Remove(descriptor);

            // Add SQLite in-memory (or LocalDB — see Lesson 3)
            services.AddDbContext<AppDbContext>(opts =>
                opts.UseSqlite($"Data Source={_dbName};Mode=Memory;Cache=Shared"));

            // ── Replace external services with test fakes ──────────────────
            // Remove production blob storage
            services.RemoveAll<IBlobStorageService>();
            services.AddSingleton<IBlobStorageService, FakeBlobStorageService>();

            // Remove production email service
            services.RemoveAll<IEmailService>();
            services.AddSingleton<IEmailService, FakeEmailService>();

            // Override JWT config for tests (use test-only key)
            services.Configure<JwtOptions>(opts =>
            {
                opts.Key      = "test-signing-key-long-enough-for-hmac-256";
                opts.Issuer   = "test-issuer";
                opts.Audience = "test-audience";
            });
        });

        builder.UseEnvironment("Test");
    }

    // ── IAsyncLifetime — create and seed database once per factory ────────
    public async Task InitializeAsync()
    {
        using var scope = Services.CreateScope();
        var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
        await db.Database.EnsureCreatedAsync();  // creates schema from migrations
        await SeedTestDataAsync(db);
    }

    private static async Task SeedTestDataAsync(AppDbContext db)
    {
        // Seed a known admin user for auth tests
        var adminUser = new ApplicationUser
        {
            Id            = "admin-test-user-id",
            UserName      = "admin@test.com",
            NormalizedUserName = "ADMIN@TEST.COM",
            Email         = "admin@test.com",
            NormalizedEmail = "ADMIN@TEST.COM",
            DisplayName   = "Test Admin",
            EmailConfirmed = true,
        };
        var hasher   = new PasswordHasher<ApplicationUser>();
        adminUser.PasswordHash = hasher.HashPassword(adminUser, "Admin!123Test");
        db.Users.Add(adminUser);
        await db.SaveChangesAsync();
    }

    public new Task DisposeAsync()
    {
        // Delete the test database file
        var conn = new SqliteConnection($"Data Source={_dbName};Mode=Memory;Cache=Shared");
        conn.Close();
        return Task.CompletedTask;
    }

    // ── Factory helpers for creating test clients ─────────────────────────
    public HttpClient CreateAuthenticatedClient(string userId = "admin-test-user-id",
                                                 string[] roles = null!)
    {
        roles ??= ["Admin"];
        var token  = JwtTestHelper.GenerateToken(userId, roles,
            key: "test-signing-key-long-enough-for-hmac-256",
            issuer: "test-issuer", audience: "test-audience");
        var client = CreateClient();
        client.DefaultRequestHeaders.Authorization =
            new AuthenticationHeaderValue("Bearer", token);
        return client;
    }
}

// ── Usage in test class ────────────────────────────────────────────────────
public class PostsIntegrationTests : IClassFixture<BlogAppFactory>
{
    private readonly BlogAppFactory _factory;
    private readonly HttpClient     _client;      // anonymous
    private readonly HttpClient     _adminClient; // authenticated as admin

    public PostsIntegrationTests(BlogAppFactory factory)
    {
        _factory     = factory;
        _client      = factory.CreateClient();
        _adminClient = factory.CreateAuthenticatedClient();
    }
}

Common Mistakes #

Mistake 1 — Creating WebApplicationFactory per test method (extremely slow) #

❌ Wrong — new BlogAppFactory() in each [Fact]; factory created 50 times for 50 tests; 5+ seconds per test.

✅ Correct — IClassFixture<BlogAppFactory>; factory created once; all tests share it.

Mistake 2 — Using ConfigureServices instead of ConfigureTestServices (override ignored) #

❌ Wrong — builder.ConfigureServices(services => ...); app’s later service registration overwrites the test override.

✅ Correct — builder.ConfigureTestServices(services => ...); runs after app registration; test override always wins.