A Maintainable ASP.NET Core API Structure

Main principle

A maintainable API needs clear boundaries. Controllers should not contain too much business logic. Services should not know too much about HTTP. Repositories should not decide business rules.

Suggested structure

For small and medium projects, start with:

src/
  Api/
  Application/
  Domain/
  Infrastructure/
tests/

Api receives requests and returns responses. Application handles use cases. Domain keeps entities and rules. Infrastructure talks to databases or external services.

DTOs and validation

Avoid exposing entities directly through the API. DTOs help control input and output data. Validation should run early so errors are clear and invalid data does not travel deep into business logic.

When should it become more complex?

Do not apply Clean Architecture mechanically. If the app is small, too much structure slows you down. Increase separation when the domain has more rules, more integrations, or stronger testing needs.