testing-dotnet

Imported from zdanovichnick/dotnet-pilot (skills/testing-dotnet/SKILL.md). Install upstream with npx skills add zdanovichnick/dotnet-pilot --skill testing-dotnet. Copyright stays with the author.

.NET Testing Patterns

Reference for test generation. Used by dnp-test-writer, dnp-tdd-developer-easy, dnp-tdd-developer-hard, and dnp-planner.

Test Organization

tests/
├── MyApp.UnitTests/           # Fast, isolated, mock dependencies
│   ├── Services/
│   │   └── UserServiceTests.cs
│   └── Domain/
│       └── UserTests.cs
├── MyApp.IntegrationTests/    # Slower, real dependencies
│   ├── Api/
│   │   └── UserEndpointTests.cs
│   └── Infrastructure/
│       └── UserRepositoryTests.cs
└── MyApp.ArchitectureTests/   # Optional: enforce architecture rules
    └── LayerDependencyTests.cs

xUnit Patterns

Test Class Setup

public class UserServiceTests
{
    private readonly Mock<IUserRepository> _repo;
    private readonly UserService _sut; // system under test

    public UserServiceTests()
    {
        _repo = new Mock<IUserRepository>();
        _sut = new UserService(_repo.Object);
    }
}

Naming Convention

MethodName_StateUnderTest_ExpectedBehavior

[Fact]
public async Task GetByIdAsync_WhenUserExists_ReturnsUser() { }

[Fact]
public async Task GetByIdAsync_WhenUserNotFound_ReturnsNull() { }

[Theory]
[InlineData("")]
[InlineData(null)]
public async Task CreateAsync_WithInvalidEmail_ThrowsValidationException(string? email) { }

IClassFixture for Shared Setup

public class DatabaseTests : IClassFixture<DatabaseFixture>
{
    private readonly DatabaseFixture _fixture;
    public DatabaseTests(DatabaseFixture fixture) => _fixture = fixture;
}

Integration Tests with WebApplicationFactory

public class UserEndpointTests : IClassFixture<WebApplicationFactory<Program>>
{
    private readonly HttpClient _client;

    public UserEndpointTests(WebApplicationFactory<Program> factory)
    {
        _client = factory.WithWebHostBuilder(builder =>
        {
            builder.ConfigureTestServices(services =>
            {
                // Replace real DB with in-memory
                services.RemoveAll<DbContextOptions<ApplicationDbContext>>();
                services.AddDbContext<ApplicationDbContext>(options =>
                    options.UseInMemoryDatabase("TestDb"));
            });
        }).CreateClient();
    }

    [Fact]
    public async Task CreateUser_Returns201WithLocation()
    {
        var request = new { Name = "Test", Email = "test@example.com" };
        var response = await _client.PostAsJsonAsync("/api/users", request);

        response.StatusCode.Should().Be(HttpStatusCode.Created);
        response.Headers.Location.Should().NotBeNull();
    }
}

Mocking Comparison

Test Data

AutoFixture (recommended for complex objects)

var fixture = new Fixture();
var user = fixture.Create<User>();

Builder Pattern (for domain-specific)

var user = new UserBuilder().WithEmail("test@example.com").Build();

Examples here use Moq because it is the most common. Read the test project's .csproj and mirror whatever it already references — a second mocking library in one solution is debt.

Choosing a Tier

Confidence per test is not uniform. An integration test through WebApplicationFactory exercises real DI, middleware, and routing; a unit test proves one method handles one case. Prefer the highest tier that is still fast and deterministic for the behavior in question.

Mocks

A test that only asserts on its mocks proves nothing. Litmus: delete every Verify(), Received(), and MustHaveHappened() call. If nothing is left that would fail when the implementation breaks, the test is measuring its own setup.

Preference order:

1. Real, if it is controllable and fast — TestServer, IMemoryCache, a real filesystem with IDisposable cleanup.
2. A container — Testcontainers for SQL Server, Redis, RabbitMQ. Slower, but the behavior is the behavior.
3. A mock — only when the dependency is non-deterministic, costly (a paid API), or slow.

A mock is a claim about a contract, so verify the claim before writing it: read the real signature (mcp__roslyn__get_class_outline) for return types and nullability, and read the implementation for the values it actually returns. A mock returning "PENDING" where the real code returns OrderStatus.Pending hides the bug it was supposed to expose.

Boundary Coverage

Every boundary the feature crosses wants at least one test with the real implementation behind it. Mock-only coverage at a boundary means nothing has proven the integration works.