RESTful API Design

Software testing is one of the most undervalued and underinvestigated topics in software education, yet it is universally considered a hallmark of professional engineering practice. Applications with comprehensive automated test suites can be refactored with confidence, deployed more frequently, and maintained more reliably over their lifetime. The cost of fixing a bug in production is orders of magnitude higher than catching it in a unit test during development. .NET Core provides a rich testing ecosystem — xUnit, NUnit, MSTest, Moq, FluentAssertions, Testcontainers, and integration testing infrastructure — that supports the full testing pyramid from fast unit tests to end-to-end integration tests. Unit testing isolates a single unit of code — typically a class or method — and tests it in complete isolation from its dependencies. In .NET Core, xUnit is the most widely adopted unit testing framework, designed specifically with .NET Core in mind and used by the Microsoft team internally. Test methods are marked with [Fact] (unconditional tests) or [Theory] with [InlineData] (parameterized tests that run with different input values). The Arrange-Act-Assert (AAA) pattern structures each test: set up preconditions (Arrange), execute the code under test (Act), and verify the results (Assert). Moq is the standard mocking library for creating test doubles that simulate dependencies like database repositories, email services, and external APIs, ensuring unit tests remain fast and deterministic. Clean Architecture — also known as Hexagonal Architecture or Ports and Adapters — is a software design philosophy that places business logic at the center of the application, completely decoupled from infrastructure concerns like databases, web frameworks, and external services. The dependency rule states that source code dependencies must point inward — outer layers can depend on inner layers, but inner layers must not depend on outer layers. In practice, this means domain entities and business logic have zero references to Entity Framework, ASP.NET Core, or any specific database library. The infrastructure layer implements repository interfaces defined in the domain layer, and dependency injection wires them together at runtime. This separation makes unit testing dramatically easier (business logic can be tested without a database), enables infrastructure swaps without changing business logic, and creates codebases that remain comprehensible as they grow. Integration testing verifies that multiple components work correctly together. ASP.NET Core provides the WebApplicationFactory<T> class for integration testing, which creates an in-memory version of your application that can be tested with a real HTTP client. Testcontainers-dotnet spins up real Docker containers for databases and external services during tests, ensuring integration tests run against genuine infrastructure rather than fakes. This combination supports a testing strategy where unit tests verify business logic, integration tests verify the application wires together correctly, and end-to-end tests verify critical user journeys.

REST (Representational State Transfer) is an architectural style for designing networked applications. It uses standard HTTP methods and status codes.