Clean Code Architecture C#: คู่มือฉบับสมบูรณ์และคำถามสัมภาษณ์ 2026

public class OrderService
{
    private readonly IOrderRepository _orderRepository;
    private readonly INotificationService _notificationService;

    public OrderService(
        IOrderRepository orderRepository,
        INotificationService notificationService)
    {
        _orderRepository = orderRepository;
        _notificationService = notificationService;
    }

    public async Task<Order> CreateOrderAsync(CreateOrderRequest request)
    {
        // Validates and creates the domain entity
        var order = Order.Create(request.CustomerId, request.Items);
        
        // Persists through the repository abstraction
        await _orderRepository.AddAsync(order);
        
        // Notifies through a separate service
        await _notificationService.SendOrderConfirmationAsync(order);
        
        return order;
    }
}

public interface IPaymentProcessor
{
    string PaymentMethod { get; }
    Task<PaymentResult> ProcessAsync(Payment payment);
}

// StripePaymentProcessor.cs
public class StripePaymentProcessor : IPaymentProcessor
{
    public string PaymentMethod => "Stripe";
    
    public async Task<PaymentResult> ProcessAsync(Payment payment)
    {
        // Stripe-specific implementation
        var charge = await _stripeClient.CreateChargeAsync(payment.Amount);
        return new PaymentResult(charge.Id, charge.Status == "succeeded");
    }
}

// PaymentService.cs
public class PaymentService
{
    private readonly IEnumerable<IPaymentProcessor> _processors;

    public PaymentService(IEnumerable<IPaymentProcessor> processors)
    {
        _processors = processors;
    }

    public async Task<PaymentResult> ProcessPaymentAsync(
        Payment payment, string method)
    {
        var processor = _processors
            .FirstOrDefault(p => p.PaymentMethod == method)
            ?? throw new NotSupportedException($"Payment method {method} not supported");
        
        return await processor.ProcessAsync(payment);
    }
}

public class Customer
{
    public Guid Id { get; private set; }
    public string Email { get; private set; }
    public CustomerStatus Status { get; private set; }

    private Customer() { } // EF Core constructor

    public static Customer Create(string email)
    {
        if (string.IsNullOrWhiteSpace(email))
            throw new DomainException("Email cannot be empty");
        
        return new Customer
        {
            Id = Guid.NewGuid(),
            Email = email.ToLowerInvariant(),
            Status = CustomerStatus.Active
        };
    }

    public void Deactivate()
    {
        if (Status == CustomerStatus.Inactive)
            throw new DomainException("Customer already inactive");
        
        Status = CustomerStatus.Inactive;
    }
}

var builder = WebApplication.CreateBuilder(args);

// Domain services - typically transient
builder.Services.AddTransient<IOrderValidator, OrderValidator>();

// Application services - scoped per request
builder.Services.AddScoped<IOrderService, OrderService>();

// Infrastructure - scoped to share DbContext
builder.Services.AddScoped<IOrderRepository, OrderRepository>();
builder.Services.AddScoped<IUnitOfWork, UnitOfWork>();

// External clients - singleton with HttpClient pooling
builder.Services.AddHttpClient<IPaymentGateway, StripeGateway>(client =>
{
    client.BaseAddress = new Uri("https://api.stripe.com/v1/");
});

// Keyed services for multiple implementations (.NET 8+)
builder.Services.AddKeyedScoped<INotificationService, EmailNotificationService>("email");
builder.Services.AddKeyedScoped<INotificationService, SmsNotificationService>("sms");

var app = builder.Build();

public interface IOrderRepository
{
    Task<Order?> GetByIdAsync(Guid id, CancellationToken ct = default);
    Task<IReadOnlyList<Order>> GetByCustomerAsync(Guid customerId, CancellationToken ct = default);
    Task AddAsync(Order order, CancellationToken ct = default);
    void Update(Order order);
}

// Infrastructure/Repositories/OrderRepository.cs
public class OrderRepository : IOrderRepository
{
    private readonly ApplicationDbContext _context;

    public OrderRepository(ApplicationDbContext context)
    {
        _context = context;
    }

    public async Task<Order?> GetByIdAsync(Guid id, CancellationToken ct = default)
    {
        return await _context.Orders
            .Include(o => o.Items)
            .FirstOrDefaultAsync(o => o.Id == id, ct);
    }

    public async Task<IReadOnlyList<Order>> GetByCustomerAsync(
        Guid customerId, CancellationToken ct = default)
    {
        return await _context.Orders
            .Where(o => o.CustomerId == customerId)
            .OrderByDescending(o => o.CreatedAt)
            .ToListAsync(ct);
    }

    public async Task AddAsync(Order order, CancellationToken ct = default)
    {
        await _context.Orders.AddAsync(order, ct);
    }

    public void Update(Order order)
    {
        _context.Orders.Update(order);
    }
}

public class OrderServiceTests
{
    private readonly IOrderRepository _orderRepository;
    private readonly INotificationService _notificationService;
    private readonly OrderService _sut;

    public OrderServiceTests()
    {
        _orderRepository = Substitute.For<IOrderRepository>();
        _notificationService = Substitute.For<INotificationService>();
        _sut = new OrderService(_orderRepository, _notificationService);
    }

    [Fact]
    public async Task CreateOrderAsync_ValidRequest_PersistsAndNotifies()
    {
        // Arrange
        var request = new CreateOrderRequest(
            CustomerId: Guid.NewGuid(),
            Items: new[] { new OrderItemDto("SKU-001", 2, 29.99m) });

        // Act
        var order = await _sut.CreateOrderAsync(request);

        // Assert
        await _orderRepository.Received(1).AddAsync(Arg.Any<Order>());
        await _notificationService.Received(1)
            .SendOrderConfirmationAsync(Arg.Is<Order>(o => o.Id == order.Id));
    }

    [Fact]
    public async Task CreateOrderAsync_EmptyItems_ThrowsDomainException()
    {
        // Arrange
        var request = new CreateOrderRequest(
            CustomerId: Guid.NewGuid(),
            Items: Array.Empty<OrderItemDto>());

        // Act & Assert
        await Assert.ThrowsAsync<DomainException>(
            () => _sut.CreateOrderAsync(request));
    }
}

public class CachingOrderRepository : IOrderRepository
{
    private readonly IOrderRepository _inner;
    private readonly IDistributedCache _cache;

    public CachingOrderRepository(
        IOrderRepository inner,
        IDistributedCache cache)
    {
        _inner = inner;
        _cache = cache;
    }

    public async Task<Order?> GetByIdAsync(Guid id, CancellationToken ct = default)
    {
        var cacheKey = $"order:{id}";
        var cached = await _cache.GetStringAsync(cacheKey, ct);
        
        if (cached is not null)
            return JsonSerializer.Deserialize<Order>(cached);
        
        var order = await _inner.GetByIdAsync(id, ct);
        
        if (order is not null)
        {
            await _cache.SetStringAsync(
                cacheKey,
                JsonSerializer.Serialize(order),
                new DistributedCacheEntryOptions
                {
                    AbsoluteExpirationRelativeToNow = TimeSpan.FromMinutes(5)
                },
                ct);
        }
        
        return order;
    }

    // Other methods delegate to _inner
}

src/
├── MyApp.Domain/
│   ├── Entities/
│   ├── ValueObjects/
│   ├── Events/
│   └── Exceptions/
├── MyApp.Application/
│   ├── Common/
│   │   ├── Interfaces/
│   │   └── Behaviors/
│   ├── Orders/
│   │   ├── Commands/
│   │   ├── Queries/
│   │   └── EventHandlers/
│   └── Customers/
├── MyApp.Infrastructure/
│   ├── Persistence/
│   ├── Services/
│   └── Configuration/
└── MyApp.WebApi/
    ├── Controllers/
    ├── Filters/
    └── Middleware/

public record OrderDto(
    Guid Id,
    Guid CustomerId,
    IReadOnlyList<OrderItemDto> Items,
    decimal Total,
    DateTime CreatedAt);

public record OrderItemDto(
    string Sku,
    int Quantity,
    decimal UnitPrice);

// Before: Long method doing multiple things
public decimal CalculateInvoiceTotal(Invoice invoice)
{
    decimal subtotal = 0;
    foreach (var item in invoice.Items)
    {
        subtotal += item.Quantity * item.UnitPrice;
    }
    
    decimal discount = 0;
    if (invoice.Customer.IsPreferred)
    {
        discount = subtotal * 0.1m;
    }
    
    decimal tax = (subtotal - discount) * 0.2m;
    
    return subtotal - discount + tax;
}

// After: Small methods with single responsibilities
public decimal CalculateInvoiceTotal(Invoice invoice)
{
    var subtotal = CalculateSubtotal(invoice.Items);
    var discount = CalculateDiscount(subtotal, invoice.Customer);
    var tax = CalculateTax(subtotal - discount);
    
    return subtotal - discount + tax;
}

private decimal CalculateSubtotal(IEnumerable<InvoiceItem> items)
    => items.Sum(item => item.Quantity * item.UnitPrice);

private decimal CalculateDiscount(decimal subtotal, Customer customer)
    => customer.IsPreferred ? subtotal * 0.1m : 0;

private decimal CalculateTax(decimal taxableAmount)
    => taxableAmount * 0.2m;