Epoch 2 – Refactor to Clean Architecture

Business Story

The initial Product Service has been successfully created as a single ASP.NET Core Web API project.

As the project grows, placing all source code inside a single project will make it difficult to maintain, test, and scale.

The architecture team has decided to refactor the service into a Clean Architecture, separating responsibilities into multiple projects.

Learning Objectives

After completing this epoch, students will be able to:

• Understand the principles of Clean Architecture.
• Create multiple class library projects using the .NET CLI.
• Organize a solution into layers.
• Configure project references.
• Apply the Dependency Rule.
• Build a scalable foundation for future development.

Target Architecture

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ProductService

                +----------------------+
                |   ProductService.API |
                +----------+-----------+
                           |
                           v
                +----------------------+
                | ProductService.Application |
                +----------+-----------+
                           |
                           v
                +----------------------+
                | ProductService.Domain |
                +----------------------+

                           ^
                           |
                +----------------------+
                | ProductService.Infrastructure |
                +----------------------+

Responsibilities

Step 1. Remove the Default Web API Project

Move to the src folder.

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cd src

Rename the existing project.

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mv ProductService ProductService.API

Rename the project file.

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mv ProductService.API/ProductService.csproj ProductService.API/ProductService.API.csproj

Step 2. Remove the Old Project from the Solution

Return to the solution folder.

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cd ..

Remove the old project.

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dotnet sln remove src/ProductService.API/ProductService.API.csproj

Step 3. Create the Remaining Projects

Move into src.

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cd src

Create the Application project.

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dotnet new classlib -n ProductService.Application

Create the Domain project.

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dotnet new classlib -n ProductService.Domain

Create the Infrastructure project.

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dotnet new classlib -n ProductService.Infrastructure

Step 4. Add All Projects to the Solution

Return to the solution root.

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cd ..

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dotnet sln add src/ProductService.API/ProductService.API.csproj

dotnet sln add src/ProductService.Application/ProductService.Application.csproj

dotnet sln add src/ProductService.Domain/ProductService.Domain.csproj

dotnet sln add src/ProductService.Infrastructure/ProductService.Infrastructure.csproj

Verify:

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dotnet sln list

Expected output:

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src/ProductService.API/ProductService.API.csproj
src/ProductService.Application/ProductService.Application.csproj
src/ProductService.Domain/ProductService.Domain.csproj
src/ProductService.Infrastructure/ProductService.Infrastructure.csproj

Step 5. Configure Project References

API

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dotnet add src/ProductService.API/ProductService.API.csproj reference src/ProductService.Application/ProductService.Application.csproj

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dotnet add src/ProductService.API/ProductService.API.csproj reference src/ProductService.Infrastructure/ProductService.Infrastructure.csproj

Application

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dotnet add src/ProductService.Application/ProductService.Application.csproj reference src/ProductService.Domain/ProductService.Domain.csproj

Infrastructure

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dotnet add src/ProductService.Infrastructure/ProductService.Infrastructure.csproj reference src/ProductService.Application/ProductService.Application.csproj

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dotnet add src/ProductService.Infrastructure/ProductService.Infrastructure.csproj reference src/ProductService.Domain/ProductService.Domain.csproj

Dependency Graph

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               API
              /   \
             /     \
    Application   Infrastructure
           \          /
            \        /
              Domain

The Domain project does not reference any other project.

Step 6. Build the Solution

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dotnet build

Expected output:

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Build succeeded.

Step 7. Verify the Folder Structure

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src

├── ProductService.API
├── ProductService.Application
├── ProductService.Domain
└── ProductService.Infrastructure

Why Use Clean Architecture?

Without Clean Architecture

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Controllers
Database
Business Logic
Validation
Repositories
DTOs

↓

Everything inside one project

Problems:

• Hard to maintain
• Difficult to test
• Strong coupling
• Low reusability

With Clean Architecture

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Presentation

↓

Application

↓

Domain

↑

Infrastructure

Benefits:

• Clear separation of responsibilities
• Easier unit testing
• Better scalability
• Easier maintenance
• Suitable for enterprise applications

Hands-on Exercise

Complete the following tasks:

• Rename the Web API project to ProductService.API
• Create three class library projects
• Add all projects to the solution
• Configure project references
• Build the solution successfully

Definition of Done

By the end of this epoch, students should have:

• ✅ Four independent projects
• ✅ Correct project references
• ✅ Successfully built solution
• ✅ Understanding of Clean Architecture
• ✅ Ready for Domain modeling