What Invariants Are (and Why a Domain Model Is the Best Place to Enforce Them)

This article discusses the concept of invariants in domain-driven design (DDD) and argues that domain models are the best place to enforce business rules. It contrasts procedural code that scatters validation across handlers, validators, and controllers with an 'always-valid' approach where the model itself prevents invalid states. Using a C# Course class example, it shows how anemic models lead to rule drift and inconsistency, while encapsulating invariants ensures correctness regardless of how an object is loaded or modified. The article is relevant to software engineering, .NET development, and best practices in domain modeling.