Clean architecture layers

The rule

Three projects, and dependencies only point inwards. Application knows nothing about the database, HTTP or any framework. Infrastructure implements the interfaces Application declares. Presentation wires everything together and turns use cases into endpoints.

Presentation ──► Application ◄── Infrastructure
      └───────────────────────────────►┘  (startup wiring only)

Infrastructure keeps its types internal. The only public surface is AddInfrastructure(), AddInfrastructureHealthChecks() and InitialiseInfrastructure() in src/Infrastructure/DependencyInjection.cs.

Steps

1. Is it a rule about the domain (what's valid, what exists, what a use case does)? It goes in Application.
2. Is it about how data is stored or fetched? Infrastructure, behind an interface in Application (IMoviesRepository, IAuthorsRepository, IReviewsRepository).
3. Is it about how a caller talks to us (routes, status codes, JSON, tokens)? Presentation.
4. Need a new dependency between projects? Only Presentation → Application, Presentation → Infrastructure and Infrastructure → Application are allowed. Tests reference the layer they test.

Example

The handler depends on an interface, not on EF Core:

// src/Application/Movies/Queries/GetMovies/GetMoviesHandler.cs
public class GetMoviesHandler(IMoviesRepository repository) : IRequestHandler<GetMoviesQuery, Page<MovieSummary>>

EntityFrameworkMovieReviewsRepository in Infrastructure implements all three repository interfaces, and AddInfrastructure() registers it for each.

Don't

• Reference Microsoft.EntityFrameworkCore or Microsoft.AspNetCore.* from Application.
• Return database models (Infrastructure/.../Models) from the repository. Map them to Application entities first (mapping-with-mapperly).
• Put validation in an endpoint. It belongs in an Application validator (validation).
• Make an Infrastructure type public so Presentation can use it. Add what's needed to DependencyInjection.cs instead.

Check

dotnet build
grep -rn "EntityFrameworkCore\|AspNetCore" src/Application --include=*.cs --include=*.csproj   # expect nothing