Clean Architecture

Core Principles

• Dependencies point inward: outer layers depend on inner layers, never the reverse
• Domain layer has zero external dependencies — pure C#
• Application layer defines interfaces that Infrastructure implements
• Presentation layer is the composition root where DI wires everything together
• One use case per handler — Application layer orchestrates domain logic

Layer Structure

src/
  {Company}.{Domain}.Domain/           # Entities, value objects, domain events, interfaces
  {Company}.{Domain}.Application/      # Commands, queries, handlers, DTOs, validators
  {Company}.{Domain}.Infrastructure/   # EF Core, external services, file system
  {Company}.{Domain}.WebApi/           # Endpoints, middleware, DI composition root

Dependency Rules

WebApi → Infrastructure → Application → Domain
                                ↑
WebApi → Application → Domain

• Domain references nothing
• Application references Domain
• Infrastructure references Application + Domain
• WebApi references all layers (composition root)

Patterns

Domain Layer

// Domain/Entities/Order.cs
namespace {Company}.{Domain}.Domain.Entities;

public sealed class Order : BaseEntity, IAggregateRoot
{
    public string CustomerName { get; private set; } = default!;
    public OrderStatus Status { get; private set; } = OrderStatus.Draft;
    public decimal Total { get; private set; }

    private readonly List<OrderItem> _items = [];
    public IReadOnlyList<OrderItem> Items => _items.AsReadOnly();

    private Order() { } // EF Core constructor

    public static Order Create(string customerName)
    {
        var order = new Order { CustomerName = customerName };
        order.AddDomainEvent(new OrderCreatedEvent(order.Id));
        return order;
    }

    public void AddItem(Guid productId, int quantity, decimal price)
    {
        Guard.Against.NegativeOrZero(quantity);
        _items.Add(new OrderItem(productId, quantity, price));
        Total = _items.Sum(i => i.Quantity * i.Price);
    }
}

// Domain/Interfaces/IOrderRepository.cs
public interface IOrderRepository
{
    Task<Order?> FindAsync(Guid id, CancellationToken ct = default);
    Task<List<Order>> ListAsync(CancellationToken ct = default);
    void Add(Order order);
}

// Domain/Interfaces/IUnitOfWork.cs
public interface IUnitOfWork
{
    Task<int> SaveChangesAsync(CancellationToken ct = default);
}

Application Layer

// Application/Orders/Commands/CreateOrder/CreateOrderCommand.cs
namespace {Company}.{Domain}.Application.Orders.Commands.CreateOrder;

public sealed record CreateOrderCommand(string CustomerName) : IRequest<Guid>;

// Application/Orders/Commands/CreateOrder/CreateOrderCommandHandler.cs
internal sealed class CreateOrderCommandHandler(
    IOrderRepository repository,
    IUnitOfWork unitOfWork) : IRequestHandler<CreateOrderCommand, Guid>
{
    public async Task<Guid> Handle(
        CreateOrderCommand request, CancellationToken ct)
    {
        var order = Order.Create(request.CustomerName);
        repository.Add(order);
        await unitOfWork.SaveChangesAsync(ct);
        return order.Id;
    }
}

// Application/Orders/Commands/CreateOrder/CreateOrderCommandValidator.cs
public sealed class CreateOrderCommandValidator
    : AbstractValidator<CreateOrderCommand>
{
    public CreateOrderCommandValidator()
    {
        RuleFor(x => x.CustomerName).NotEmpty().MaximumLength(200);
    }
}

Infrastructure Layer

// Infrastructure/Persistence/AppDbContext.cs
namespace {Company}.{Domain}.Infrastructure.Persistence;

internal sealed class AppDbContext(
    DbContextOptions<AppDbContext> options) : DbContext(options), IUnitOfWork
{
    public DbSet<Order> Orders => Set<Order>();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.ApplyConfigurationsFromAssembly(
            typeof(AppDbContext).Assembly);
    }
}

// Infrastructure/Persistence/Repositories/OrderRepository.cs
internal sealed class OrderRepository(AppDbContext db) : IOrderRepository
{
    public async Task<Order?> FindAsync(Guid id, CancellationToken ct)
        => await db.Orders
            .Include(o => o.Items)
            .FirstOrDefaultAsync(o => o.Id == id, ct);

    public async Task<List<Order>> ListAsync(CancellationToken ct)
        => await db.Orders.ToListAsync(ct);

    public void Add(Order order) => db.Orders.Add(order);
}

// Infrastructure/DependencyInjection.cs
public static class DependencyInjection
{
    public static IServiceCollection AddInfrastructure(
        this IServiceCollection services, IConfiguration configuration)
    {
        services.AddDbContext<AppDbContext>(options =>
            options.UseSqlServer(
                configuration.GetConnectionString("Default")));

        services.AddScoped<IUnitOfWork>(sp =>
            sp.GetRequiredService<AppDbContext>());
        services.AddScoped<IOrderRepository, OrderRepository>();

        return services;
    }
}

WebApi Layer (Composition Root)

// WebApi/Program.cs
var builder = WebApplication.CreateBuilder(args);

builder.Services
    .AddApplicationServices()
    .AddInfrastructure(builder.Configuration);

builder.Services.AddEndpointsApiExplorer();
builder.Services.AddOpenApi();

var app = builder.Build();

app.MapOpenApi();
app.MapEndpointGroups();
app.Run();

Anti-Patterns

• Domain layer referencing EF Core or any infrastructure package
• Application layer creating infrastructure objects (e.g., new SqlConnection)
• Business logic in controllers or endpoints
• Returning domain entities from API endpoints (use DTOs)
• Infrastructure interfaces defined in Infrastructure (should be in Application/Domain)
• Circular references between layers

Detect Existing Patterns

1. Look for separate projects named Domain, Application, Infrastructure, WebApi/Api/Presentation
2. Check project references: Infrastructure should reference Application, not vice versa
3. Look for repository interfaces in Domain, implementations in Infrastructure
4. Check for IUnitOfWork interface in Domain or Application layer
5. Verify domain entities have no using statements for EF Core or infrastructure

Adding to Existing Project

1. Create the four projects with correct naming: {Company}.{Domain}.{Layer}
2. Move entities to Domain — remove any infrastructure dependencies
3. Extract interfaces for repositories and external services into Domain/Application
4. Move handlers to Application — ensure they depend only on interfaces
5. Move implementations to Infrastructure — EF Core, repositories, external clients
6. Set up composition root in WebApi with DI registration extension methods
7. Verify project references follow the dependency rule

Decision Guide

Question	Answer
Where do entities go?	Domain
Where do repository interfaces go?	Domain (per aggregate root)
Where do DTOs go?	Application
Where do MediatR handlers go?	Application
Where does DbContext go?	Infrastructure
Where does DI registration go?	Each layer has its own + WebApi composes
Where does validation go?	Application (FluentValidation)

References

• https://learn.microsoft.com/en-us/dotnet/architecture/modern-web-apps-azure/common-web-application-architectures#clean-architecture
• https://blog.cleancoder.com/uncle-bob/2012/08/13/the-clean-architecture.html