🧅 Understanding the Onion Architecture: A Clean Approach to Building Scalable Applications

By Tareq Marji -

 

When building modern, maintainable software applications, how you organize your code matters a lot. Over time, developers have learned that tightly coupled systems become hard to maintain, test, and evolve.

Enter the Onion Architecture — a clean, layered architectural pattern that addresses many of these challenges by enforcing strict separation of concerns.

📌 What is Onion Architecture?

The Onion Architecture, introduced by Jeffrey Palermo, is an architectural pattern designed to build maintainable, testable, and loosely coupled applications.

Its core idea:
The application is structured in concentric layers (like an onion), where dependencies always point inward toward the core.

🧅 Layers of Onion Architecture

Here’s a quick breakdown of the typical layers from inside out:

1. Core (Domain) Layer

  • Holds the business logic, domain entities, and domain services.
  • No dependencies on other layers.
  • Contains pure domain models (POCOs).
  • Example: Customer, Order, Invoice, domain rules.

2. Application Layer

  • Contains application logic and service interfaces.
  • Coordinates tasks and delegates to the domain.
  • Defines interfaces for infrastructure dependencies (e.g., repositories).
  • Example: ICustomerService, OrderProcessor.

3. Infrastructure Layer

  • Implements interfaces defined in the application layer.
  • Deals with database access, file system, external APIs, messaging.
  • Depends on Application and Domain layers.
  • Example: Entity Framework repositories, file storage classes.

4. Presentation Layer (UI / API)

  • Handles user interaction or API endpoints.
  • Depends on Application and Domain layers.
  • Examples: ASP.NET Core MVC, Razor Pages, Web API.

🔄 Dependency Rule

The Dependency Rule in Onion Architecture says:

Source code dependencies can only point inwards.
Outer layers can depend on inner layers, but inner layers cannot depend on outer layers.

This keeps the domain layer pure and independent.

🛠️ Why Use Onion Architecture?

  • Maintainability: Easy to update or replace outer layers without affecting core logic.
  • Testability: Core logic is independent, easy to test in isolation.
  • Flexibility: Swap out UI, database, or external systems easily.
  • Separation of Concerns: Each layer has a distinct responsibility.
  • Decoupling: Reduces tight coupling common in traditional layered architectures.

🧩 Onion Architecture in .NET Core — Example Structure

graphql

CopyEdit

/MyApp

|-- /MyApp.Domain         # Core business models, interfaces

|-- /MyApp.Application    # Application services, interfaces

|-- /MyApp.Infrastructure # EF Core, database, external services

|-- /MyApp.API            # ASP.NET Core Web API (presentation)

Sample Domain Entity (MyApp.Domain):

csharp

CopyEdit

public class Customer

{

    public Guid Id { get; set; }

    public string Name { get; set; }

}

Application Service Interface (MyApp.Application):

csharp

CopyEdit

public interface ICustomerService

{

    Task<Customer> GetCustomerAsync(Guid id);

}

Infrastructure Implementation (MyApp.Infrastructure):

csharp

CopyEdit

public class CustomerRepository : ICustomerRepository

{

    private readonly AppDbContext _context;

 

    public CustomerRepository(AppDbContext context)

    {

        _context = context;

    }

 

    public async Task<Customer> GetByIdAsync(Guid id)

    {

        return await _context.Customers.FindAsync(id);

    }

}

API Controller (MyApp.API):

csharp

CopyEdit

[ApiController]

[Route("api/[controller]")]

public class CustomersController : ControllerBase

{

    private readonly ICustomerService _customerService;

 

    public CustomersController(ICustomerService customerService)

    {

        _customerService = customerService;

    }

 

    [HttpGet("{id}")]

    public async Task<IActionResult> Get(Guid id)

    {

        var customer = await _customerService.GetCustomerAsync(id);

        if (customer == null) return NotFound();

 

        return Ok(customer);

    }

}

🔧 How to Get Started with Onion Architecture in .NET Core

  1. Create separate projects for Domain, Application, Infrastructure, and API.
  2. Define your domain models and interfaces inside the Domain project.
  3. Add application logic and service interfaces in Application project.
  4. Implement interfaces and external dependencies in Infrastructure.
  5. Build the API or UI in the outermost layer referencing Application and Domain.
  6. Use Dependency Injection to inject services from outer layers into API/UI.
  7. Write unit tests targeting Domain and Application layers (no infrastructure).

⚠️ Common Pitfalls

  • Violating the Dependency Rule by letting inner layers reference outer layers.
  • Overloading the domain model with infrastructure concerns.
  • Tight coupling between UI and database layers.
  • Forgetting to abstract infrastructure dependencies behind interfaces.

🚀 Final Thoughts

The Onion Architecture provides a robust way to build scalable, testable, and maintainable .NET Core applications. By isolating your core business logic and making dependencies explicit and one-directional, you create clean code that’s easier to evolve as requirements change.

 

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