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Introduced by Jeffrey Palermo, Onion Architecture uses the Dependency Inversion Principle to ensure your domain logic is the center of your universe.
- **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).
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.
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."
Quizzes linked to this course—pass to earn certificates.
On this page
1. Layers of the Onion 2. The Dependency Rule 3. Architect Insight