dotnet-testing

When to activate

• Writing a unit test or integration test in .NET / C#
• "What should I mock?" in a .NET testing context
• Setting up a test project for the first time in a solution
• Configuring WebApplicationFactory for ASP.NET Core API tests
• Implementing database isolation for integration tests
• Testing a MediatR Command, Query, or Handler
• Testing FluentValidation validators
• Setting up shared test infrastructure (Docker containers, DB migrations)
• Any question about xUnit fixtures, test collections, or parallel execution
• "How do I test this without hitting the real database?"

Rules — Non-negotiable

1. One Act per test. Strict AAA pattern: one action triggers one outcome. A test that does two things hides which one failed.

2. Mock only external/unmanaged dependencies. Moq is for third-party APIs, email services, payment gateways — not for repositories or domain services. Test those with real implementations.

3. Integration tests use real databases. In-memory providers (UseInMemoryDatabase) do not enforce foreign keys, SQL constraints, or stored procedures. Use a real SQL Server instance (Docker) for integration tests.

4. Every test leaves zero side effects. Use transaction rollback or database reset between tests. Never rely on test execution order.

5. Unit tests have zero infrastructure. No database, no file system, no HTTP calls, no environment variables in unit tests. If you need them, it's an integration test.

Test Project Structure

solution/
├── src/
│   ├── Domain/
│   ├── Application/
│   ├── Infrastructure/
│   └── API/
└── tests/
    ├── Domain.UnitTests/          # Pure domain logic — no mocks needed usually
    ├── Application.UnitTests/     # Handlers, validators — mock output ports
    ├── Infrastructure.IntegrationTests/  # Repositories against real DB
    └── API.IntegrationTests/      # Full HTTP stack via WebApplicationFactory

Each test project mirrors the production project it covers. Test class names: {ClassUnderTest}Tests.cs Test method names: {MethodName}_{Scenario}_{ExpectedResult}