Clean Architecture: Onion Layers in Cooking

Software projects can quickly turn into a mess if there are no rules about how to organise the code. As your application grows, it becomes hard to change features, fix bugs, or add new ideas. You might find that changing one part breaks something completely unrelated. This is where Clean Architecture comes in — it gives your project clear rules about how to structure code so everything stays tidy and flexible, no matter how big it gets.

Clean Architecture is a way of organising your code into layers. Each layer has its own job and clear boundaries. The main idea is to protect the most important parts of your system (like your business rules) from outside changes, like user interfaces or databases. If you need to swap your database, web framework, or even the UI, your core logic stays safe and untouched. This makes your project easier to maintain, test, and grow.

Real-Life Analogy: Onion Layers in Cooking

Imagine an onion in your kitchen. An onion has many layers — you can't reach the middle without going through the outside layers first. The core is the most important and delicate part, so it is surrounded by protective layers. Each layer shields the next, and you must pass through each one in order.

Benefits of Clean Architecture

Clean Architecture isn't just about making code look nice. It gives your application structure and helps avoid a big, tangled mess of dependencies. You get flexibility, better testability, and the ability to swap out technology without rewriting everything from scratch. The system becomes easier to understand for new developers, and you can add features with less risk.

• Separation of concerns: Each layer only handles its specific job.
• Easy to test: You can test core business rules without worrying about databases or UI.
• Independent of frameworks: Change your tools or libraries without touching the core logic.
• Maintainability: It's easier to add, change, or remove features.
• Scalability: Helps big teams work on different layers at the same time.

What to Implement

To follow Clean Architecture, divide your code into clear layers, usually like this:

• Core Layer: Contains Entities (business models) and Use Cases (application logic). No outside dependencies here.
• Interface Layer: Also called Application Services or Ports. Defines how the outside world can interact with the core — for example, through interfaces or service contracts.
• Infrastructure Layer: Handles things like database access, web APIs, and external services. This layer depends on the core, not the other way around.
• Presentation Layer: The UI or API controllers. These show information to the user or receive input. They can be swapped out without touching the business logic.

The key rule: Dependencies only point inward — outer layers can use inner layers, but not the other way around.

How It Works in C#

Here's a simplified example of Clean Architecture in C#. We'll show a simple case for a To-Do app.

// Core Layer
public class TodoItem {
    public int Id { get; set; }
    public string Title { get; set; }
    public bool IsCompleted { get; set; }
}

public interface ITodoRepository {
    void Add(TodoItem item);
    List<TodoItem> GetAll();
}

// Use Case
public class AddTodo {
    private readonly ITodoRepository _repo;
    public AddTodo(ITodoRepository repo) {
        _repo = repo;
    }
    public void Execute(TodoItem item) {
        _repo.Add(item);
    }
}

// Infrastructure Layer
public class InMemoryTodoRepository : ITodoRepository {
    private List<TodoItem> _items = new List<TodoItem>();
    public void Add(TodoItem item) => _items.Add(item);
    public List<TodoItem> GetAll() => _items;
}

// Presentation Layer (e.g. API Controller)
public class TodoController {
    private readonly AddTodo _addTodo;
    public TodoController(AddTodo addTodo) {
        _addTodo = addTodo;
    }
    public void Post(TodoItem item) {
        _addTodo.Execute(item);
    }
}

In this example, the core (entities and use cases) knows nothing about the database or the UI. You can replace the repository or controller easily, and the business rules always stay safe at the centre.

When Should You Use It?

Clean Architecture is a great choice when your application is expected to grow, will have lots of features, or must last for years. It is especially useful for enterprise or business-critical software, where changes happen often and bugs can be costly. If you want a long-lasting project with a clear structure that can survive technology changes, this approach is ideal.

• When building business applications with lots of rules and logic
• When working with large teams or needing clear responsibility boundaries
• When your app might change database, UI, or frameworks in the future
• When writing software that should be easy to test

When Not to Use It: If you're making a tiny project, a simple script, or a quick prototype, Clean Architecture may feel like too much overhead. For small, throwaway apps, a simpler structure is fine.

Where Is It Used in the Real World?

Many big companies and open-source projects use Clean Architecture or similar ideas to keep their software manageable. You'll see it in banking systems, e-commerce platforms, medical software, and enterprise web applications. Even some game studios use it to keep the game logic independent of the engine or graphics.

• Banking and finance software
• E-commerce platforms
• Healthcare record systems
• Large-scale APIs
• Any project with strict business rules

Final Thoughts

Clean Architecture is a smart way to build reliable, flexible, and long-lasting software. It protects your business logic, makes your application easier to test and maintain, and helps you avoid technical debt as your project grows. Like an onion, every layer adds protection and purpose, ensuring that the core of your system is always safe, no matter what changes around it.