Week 4 Part II · API and Persistence

ASP.NET Core Web API

Instructor lesson plan: lecture (2 h) and practice (2 h).

Learning objectives

Build ASP.NET Core endpoints using controllers or Minimal APIs.
Explain routing, middleware, dependency injection, and OpenAPI generation.
Design DTOs that protect the domain model from API shape changes.
Add validation and consistent response contracts.

Tools this week

ASP.NET CoreSwagger / OpenAPIFluentValidationProblemDetails

🎓Lecture · 2 hours

0:00-0:15	15 min	HTTP and API design recapResources, verbs, status codes, idempotency, and JSON contracts.
0:15-0:40	25 min	ASP.NET Core request pipeline Program.cs, middleware order, routing, endpoint selection. Dependency injection and lifetime choices.
0:40-1:05	25 min	Controllers and Minimal APIsWhen each style is appropriate and how to keep the API layer thin.
1:05-1:15	10 min	Swagger and OpenAPIAPI documentation as a contract with clients.
1:15-1:25	10 min	Break
1:25-1:50	25 min	DTOs and validation Request and response models versus domain entities. Validation boundaries and error shapes.
1:50-2:00	10 min	Project previewFirst project endpoints needed for the proposal presentation.

Common misconception to confront.

Students often think: the controller is where the business logic goes.
Set it straight: controllers translate HTTP into application calls. Business behavior belongs behind the API layer.

Check for understanding

Why use DTOs instead of returning EF entities?

DTOs protect the API contract, prevent over-posting, hide persistence details, and let the domain evolve independently.

Where does middleware run?

In the request pipeline, before and after endpoint execution depending on order and whether it calls the next component.

Key takeaways.

The API layer is a boundary, not the application core.
OpenAPI documents the contract clients rely on.
Validation and error responses should be predictable.

📚Reading & resources

ASP.NET Core Web API Official guide to API controllers and conventions.
Middleware How request processing is composed.
OpenAPI support API description and Swagger tooling.

💻Practice · 2 hours

0:00-0:20	20 min	Create API projectAdd ASP.NET Core API project to the solution and run Swagger locally.
0:20-0:55	35 min	First endpoints Create list, get-by-id, and create endpoints for a core resource. Map DTOs to application commands or service calls.
0:55-1:10	15 min	ValidationAdd request validation and inspect failure responses.
1:10-1:20	10 min	Break
1:20-1:50	30 min	OpenAPI reviewTeams review each other's API contract and name one ambiguity.
1:50-2:00	10 min	Project-integration briefPrepare API screenshots and endpoint list for Proposal Presentation.

Project integration (this week)

Add first working API endpoints.
Define request and response DTOs.
Generate OpenAPI documentation for the current API.