Implementing Clean Architecture with ASP.NET Core 8: A Practical Guide

Learn how to structure your .NET applications for maintainability, testability, and domain-centric design using Clean Architecture principles.

What is Clean Architecture?

Clean Architecture, also known as Onion Architecture or Hexagonal Architecture (Ports and Adapters), represents a domain-centric approach to application design. Unlike traditional database-centered architectures where everything eventually depends on the database, Clean Architecture ensures that business logic remains independent of infrastructure concerns.

The core principle involves organizing dependencies so that they point inward toward the domain model, rather than having outer layers like UI depending on inner layers like data access. This is achieved through the Dependency Inversion Principle, using abstractions and interfaces defined in the core domain project.

When to Use Clean Architecture

This architectural approach is particularly valuable for teams practicing Domain-Driven Design (DDD) who want to maintain focus on their domain model rather than infrastructure concerns. It's ideal for applications with complex business logic that requires extensive automated testing.

The architecture acts as a enforcement mechanism, using the compiler to prevent accidental dependencies on infrastructure from creeping into the domain model. This provides guardrails that help teams maintain architectural consistency without relying solely on developer discipline.

Core Architecture Components

The Core Project

The heart of Clean Architecture is the core project, which contains:

• Interfaces defining what needs to happen (not how)
• Domain model types including Aggregates, Entities, and Value Objects
• Domain Services for logic that doesn't fit within entities
• Domain Exceptions for specific error scenarios
• Domain Events and Handlers for decoupled workflows
• Specifications for defining reusable queries
• Validators and custom guard clauses

Use Cases Project (Optional CQRS)

For applications using Command Query Responsibility Segregation (CQRS), a separate use cases project sits between the UI and domain model. This contains:

• Commands and Command Handlers
• Queries and Query Handlers
• DTOs supporting these operations
• Application-specific behaviors

Infrastructure Project

The infrastructure layer handles implementation details:

• Persistence implementations (Entity Framework, Dapper, ADO.NET)
• API clients for third-party services
• File system access, email/SMS services
• System clock access
• Infrastructure-specific interfaces

Web Project

The web layer contains ASP.NET Core-specific components:

• Controllers or Minimal APIs/Endpoints
• Service composition and dependency wiring
• Web-specific services and interfaces
• View models (if using MVC/Razor Pages)

Practical Implementation Example

A sample task tracking system demonstrates these principles with two main aggregates: Contributors and Projects. The domain model contains business logic for operations like marking todo items as complete, while domain events handle side effects like sending notifications.

Specifications help avoid LINQ code duplication across the application by centralizing query logic in named, reusable components. Domain event handlers use dependency injection to perform infrastructure-dependent tasks without polluting the domain model.

Getting Started Template

A template is available to quickly start new projects with Clean Architecture:

This template creates the solution structure with proper project dependencies and includes test projects, saving significant setup time.

Architecture Comparison

Compared to traditional N-Tier architecture where dependencies flow from UI to Business to Data layers (all ultimately depending on the database), Clean Architecture inverts these dependencies. The core domain has no dependencies on external concerns, while infrastructure and UI depend inward on the domain abstractions.

This inversion enables easier testing (domain logic can be tested without infrastructure) and more flexible replacement of implementation details (changing databases or UI frameworks without affecting business logic).