การฉีดผ่าน Constructor และอินเทอร์เฟซ

public interface IProductRepository
{
    Task<Product?> GetByIdAsync(int id);
    Task<IEnumerable<Product>> GetAllAsync();
    Task AddAsync(Product product);
}

public class SqlProductRepository : IProductRepository
{
    private readonly AppDbContext _db;

    public SqlProductRepository(AppDbContext db) => _db = db;

    public async Task<Product?> GetByIdAsync(int id)
        => await _db.Products.FindAsync(id);

    public async Task<IEnumerable<Product>> GetAllAsync()
        => await _db.Products.ToListAsync();

    public async Task AddAsync(Product product)
    {
        _db.Products.Add(product);
        await _db.SaveChangesAsync();
    }
}

public class ProductService
{
    private readonly IProductRepository _repo;
    private readonly ILogger<ProductService> _logger;

    public ProductService(
        IProductRepository repo,
        ILogger<ProductService> logger)
    {
        _repo = repo;
        _logger = logger;
    }

    public async Task<Product?> GetProductAsync(int id)
    {
        _logger.LogInformation("Fetching product {Id}", id);
        return await _repo.GetByIdAsync(id);
    }
}

builder.Services.AddDbContext<AppDbContext>(opt =>
    opt.UseSqlite("Data Source=app.db"));

builder.Services.AddScoped<IProductRepository, SqlProductRepository>();
builder.Services.AddScoped<ProductService>();

// Using NSubstitute or Moq in a unit test
var mockRepo = Substitute.For<IProductRepository>();
mockRepo.GetByIdAsync(1).Returns(new Product { Id = 1, Name = "Widget" });

var logger = Substitute.For<ILogger<ProductService>>();
var svc = new ProductService(mockRepo, logger);

var result = await svc.GetProductAsync(1);
Assert.Equal("Widget", result?.Name);

public class OrderService
{
    public OrderService(
        IOrderRepository orders,
        IProductRepository products,
        IEmailSender email,
        ILogger<OrderService> logger)
    {
        // all injected by DI container
    }
}

// BAD
public void Process(IServiceProvider sp)
{
    var repo = sp.GetService<IProductRepository>(); // hidden dep!
}

// GOOD
public class Processor
{
    private readonly IProductRepository _repo;
    public Processor(IProductRepository repo) => _repo = repo;
}

// C# 12 primary constructor
public class ProductService(
    IProductRepository repo,
    ILogger<ProductService> logger)
{
    public async Task<Product?> GetAsync(int id)
    {
        logger.LogInformation("Getting {Id}", id);
        return await repo.GetByIdAsync(id);
    }
}

// Switch from SQL to in-memory for integration tests
if (environment.IsEnvironment("Test"))
    builder.Services.AddScoped<IProductRepository, InMemoryProductRepository>();
else
    builder.Services.AddScoped<IProductRepository, SqlProductRepository>();

[ApiController]
[Route("api/products")]
public class ProductsController : ControllerBase
{
    private readonly ProductService _svc;

    public ProductsController(ProductService svc) => _svc = svc;

    [HttpGet("{id}")]
    public async Task<IActionResult> Get(int id)
    {
        var product = await _svc.GetProductAsync(id);
        return product is null ? NotFound() : Ok(product);
    }
}