tdd

/tdd -- Red-Green-Refactor for .NET

What

Guides a strict test-driven development cycle for .NET features. Instead of writing implementation first and bolting on tests after, this command flips the order: write a failing test that defines the desired behavior, implement the minimum code to make it pass, then refactor with confidence.

Every cycle uses the .NET testing stack:

• xUnit v3 -- Test framework with [Fact] and [Theory]
• WebApplicationFactory -- Integration tests against the real HTTP pipeline
• Testcontainers -- Real databases (PostgreSQL, SQL Server) in tests
• Verify -- Snapshot testing for complex response structures
• FakeTimeProvider -- Deterministic time in tests

When

• User says "TDD", "test-driven", "let's TDD this", "write the test first"
• Building a new feature with clear acceptance criteria
• Fixing a bug (write a test that reproduces the bug first, then fix)
• Adding behavior to an existing feature (test the new behavior first)
• Any time the user wants proof that code works before it ships

Skip TDD for: Trivial config changes, scaffolding without logic, documentation.

How

Cycle: Red -> Green -> Refactor

Each feature goes through one or more TDD cycles. A cycle covers one discrete behavior.

Step 1: Red -- Write the Failing Test

Write a test that describes the desired behavior. The test MUST fail because the implementation does not exist yet.

[Fact]
public async Task CreateOrder_WithValidItems_Returns201WithOrderId()
{
    // Arrange
    var client = _factory.CreateClient();
    var request = new CreateOrderRequest([
        new OrderItemRequest("SKU-001", 2, 29.99m)
    ]);

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

    // Assert — plain xUnit Assert (FluentAssertions v8+ requires a commercial license)
    Assert.Equal(HttpStatusCode.Created, response.StatusCode);
    var result = await response.Content.ReadFromJsonAsync<CreateOrderResponse>();
    Assert.NotNull(result);
    Assert.NotEqual(Guid.Empty, result.OrderId);
}