The Onion Pattern

Introduced by Jeffrey Palermo, Onion Architecture uses the Dependency Inversion Principle to ensure your domain logic is the center of your universe.

1. Layers of the Onion

- **Core (Domain):** Entities, Value Objects, and Domain Services.
- **In-Between (Application):** Interfaces (Abstractions) for the outside world.
- **Outer Ring (Infrastructure/UI):** Implementations of those interfaces (EF Core, Email Services, API Controllers).

2. The Dependency Rule

Dependencies always point **Inward**. The Core knows nothing about the Outer Rings. If the Business layer needs to save a user, it defines an IUserRepository interface. The Infrastructure layer then implements that interface. The Core stays pure and testable.

3. Architect Insight

Q: "Isn't this just more boilerplate?"

Architect Answer: "In the short term, yes. You'll write more interfaces and more projects. But in the long term, it's a lifecycle-saver. When Microsoft releases .NET 9 or you decide to move from SQL Server to PostgreSQL, you only change the **Outer Ring**. Your 10,000 lines of complex business logic in the Core remain completely untouched."