dotnet-architecture

.NET Core Layered Architecture

Layer Overview

Controller
    ↓ depends on
Application Service
    ↓ depends on
Domain Service  ←→  Repository / Proxy

Dependencies flow inward only. Inner layers must not reference outer layers. Interfaces are defined in inner layers, implemented in outer layers (Infrastructure).

---

Layer Responsibilities

Controller

• Extracts parameters from the HTTP request
• Assembles the input DTO to pass into Application Service
• Maps the result to a ViewModel (Response) for the HTTP response
• No business logic. No direct dependency on Repository, Domain Service, or any infrastructure.

Application Service

• Orchestration only — coordinates calls between Domain Services and/or Repositories
• Receives a DTO as input
• Returns a domain model when the operation represents a domain event; returns a DTO otherwise
• Contains no business rules — any conditional logic on domain state belongs in Domain Service

Two valid dependency patterns:

Scenario	Allowed dependencies
Simple CRUD / query	Application Service → Repository or Proxy directly
Domain event handling	Application Service → Domain Service(s) only

Domain Service

• Owns business rules and domain logic
• Receives and returns domain models
• May depend on Repository or Proxy interfaces (defined here, implemented in Infrastructure)

Repository

• Abstracts persistence
• Always returns an Aggregate — individual entities must not leak out of the infrastructure layer
• Interface defined in Domain layer, implementation in Infrastructure

Proxy

• Abstracts external APIs and third-party services
• Always returns a DTO
• Interface defined in Application or Domain layer, implementation in Infrastructure

---

Type Boundaries

Layer boundary	Input type	Output type
HTTP → Controller	HTTP request	—
Controller → Application Service	DTO	—
Application Service → caller (Controller)	—	Domain model (domain event) or DTO (query/simple op)
Controller → HTTP response	—	ViewModel (converted from domain model)
Domain Service → Application Service	Domain model	Domain model
Repository → any caller	—	Aggregate
Proxy → any caller	—	DTO

Key rules

• ViewModel (Request/Response) lives in the Controller layer. It is only converted to/from a domain model — never constructed from raw fields scattered across layers.
• Entities must not be returned by any public layer boundary. Repository exposes Aggregates; everything else uses domain models or DTOs.
• DTOs are passed into Application Service and returned by Proxy. They do not carry domain behavior.

---

Dependency Direction

Controller          →  IApplicationService
ApplicationService  →  IRepository | IProxy       (simple CRUD/query)
ApplicationService  →  IDomainService              (domain event)
DomainService       →  IRepository | IProxy

• Interfaces are defined in the inner layer that needs them
• Implementations live in Infrastructure
• No layer may import a namespace from a layer above it

---

Cross-cutting Concerns

Action Filter — API pre-conditions

Use Action Filters for concerns that apply before a controller action executes and are specific to HTTP/API context:

• Authentication / authorization checks
• Request-level validation (model state)
• Rate limiting, idempotency key checks

public class RequireFeatureFlagFilter : IActionFilter {
    public void OnActionExecuting(ActionExecutingContext context) {
        if (!_featureFlags.IsEnabled("NewCheckout"))
            context.Result = new StatusCodeResult(503);
    }
    public void OnActionExecuted(ActionExecutedContext context) { }
}

Middleware — global cross-cutting behavior

Use Middleware for concerns that span the entire request pipeline:

• Global exception handling (see below)
• Logging / tracing
• Correlation ID injection
• Response compression, CORS

---

Error Handling

Each layer is responsible only for throwing the appropriate exception. Catching and converting to HTTP responses is handled by a single global exception-handling middleware.

Exception conventions by layer

Exception	Thrown by	HTTP status
NotFoundException	Repository, Domain Service	404
ValidationException	Domain Service, Application Service	422
UnauthorizedException	Domain Service	403
Unhandled Exception	anywhere	500

Global exception middleware

public class ExceptionHandlingMiddleware {
    public async Task InvokeAsync(HttpContext context, RequestDelegate next) {
        try {
            await next(context);
        }
        catch (NotFoundException ex) {
            context.Response.StatusCode = 404;
            await context.Response.WriteAsJsonAsync(new { error = ex.Message });
        }
        catch (ValidationException ex) {
            context.Response.StatusCode = 422;
            await context.Response.WriteAsJsonAsync(new { error = ex.Message });
        }
        catch (Exception ex) {
            context.Response.StatusCode = 500;
            await context.Response.WriteAsJsonAsync(new { error = "Unexpected error." });
        }
    }
}

Layers throw — middleware catches. No try/catch blocks inside business logic.

---

Full Example — Simple Query

// Controller
[HttpGet("{id}")]
public async Task<OrderResponse> GetOrder(Guid id) {
    var dto = await _orderAppService.GetOrder(id);
    return OrderResponse.From(dto);           // ViewModel converted from DTO
}

// Application Service (simple query — depends on Repository directly)
public async Task<OrderDto> GetOrder(Guid id) {
    var aggregate = await _orderRepo.Get(id); // Repository returns Aggregate
    return aggregate.ToDto();
}

Full Example — Domain Event

// Controller
[HttpPost]
public async Task<OrderResponse> PlaceOrder(PlaceOrderRequest request) {
    var dto = request.ToDto();
    var order = await _orderAppService.PlaceOrder(dto);   // returns domain model
    return OrderResponse.From(order);                     // ViewModel from domain model
}

// Application Service (domain event — depends on Domain Service only)
public async Task<Order> PlaceOrder(PlaceOrderDto dto) {
    return await _orderDomainService.PlaceOrder(dto);
}

// Domain Service (business rules, depends on Repository interface)
public async Task<Order> PlaceOrder(PlaceOrderDto dto) {
    var user = await _userRepo.Get(dto.UserId);
    var products = await _productRepo.GetMany(dto.ProductIds);
    var order = Order.For(user).WithProducts(products).Confirm();
    await _orderRepo.Save(order);
    return order;
}

---

Violation Quick Reference

Symptom	Fix
Controller calls Repository directly	Route through Application Service
Application Service contains if on domain state	Move logic to Domain Service
Application Service for domain event calls Repository directly	Route through Domain Service
Entity returned from Repository	Wrap in Aggregate
Entity exposed at Controller boundary	Convert to ViewModel first
ViewModel constructed from raw fields in Service	Move conversion to domain model To...()
Business logic in Controller	Move to Domain Service
Try/catch in a Service or Domain class	Remove — let middleware handle it
API pre-condition logic inside Controller action	Move to Action Filter
Infrastructure concern (logging, CORS) in Controller	Move to Middleware
Inner layer imports outer layer namespace	Invert dependency — define interface in inner layer