dependency-injection

依赖注入

核心原则

1. 默认使用构造函数注入 — 通过构造函数注入依赖项（主构造函数可让这一过程更加简洁）。不要使用服务定位器或属性注入。
2. 谨慎匹配生命周期 — 单例绝不能依赖作用域或瞬态服务。这是最常见的 DI 错误。
3. 注册接口，解析接口 — 注册 services.AddScoped<IOrderService, OrderService>()，而不是具体类型。
4. 使用键控服务实现策略模式 — .NET 8+ 的键控服务取代了用于选择不同实现的手动工厂模式。

模式

键控服务（.NET 8+）

使用键控服务注册和解析同一接口的多个实现。

// Registration
builder.Services.AddKeyedScoped<INotificationService, EmailNotificationService>("email");
builder.Services.AddKeyedScoped<INotificationService, SmsNotificationService>("sms");
builder.Services.AddKeyedScoped<INotificationService, PushNotificationService>("push");

// Resolution via attribute
public class OrderHandler([FromKeyedServices("email")] INotificationService notifier)
{
    public async Task Handle(CreateOrder.Command command, CancellationToken ct)
    {
        // ... create order
        await notifier.SendAsync(notification, ct);
    }
}

// Resolution via IServiceProvider
public class NotificationRouter(IServiceProvider provider)
{
    public INotificationService GetService(string channel)
    {
        return provider.GetRequiredKeyedService<INotificationService>(channel);
    }
}

装饰器模式

// Base service
public interface IOrderService
{
    Task<Result<Order>> CreateAsync(CreateOrderRequest request, CancellationToken ct);
}

public class OrderService(AppDbContext db, TimeProvider clock) : IOrderService
{
    public async Task<Result<Order>> CreateAsync(CreateOrderRequest request, CancellationToken ct)
    {
        var order = Order.Create(request, clock.GetUtcNow());
        db.Orders.Add(order);
        await db.SaveChangesAsync(ct);
        return Result.Success(order);
    }
}

// Decorator — adds logging
public class LoggingOrderService(IOrderService inner, ILogger<LoggingOrderService> logger) : IOrderService
{
    public async Task<Result<Order>> CreateAsync(CreateOrderRequest request, CancellationToken ct)
    {
        logger.LogInformation("Creating order for customer {CustomerId}", request.CustomerId);
        var result = await inner.CreateAsync(request, ct);
        if (result.IsSuccess)
            logger.LogInformation("Order {OrderId} created", result.Value.Id);
        return result;
    }
}

// Registration with Scrutor
builder.Services.AddScoped<IOrderService, OrderService>();
builder.Services.Decorate<IOrderService, LoggingOrderService>();

按约定注册（Scrutor）

// Auto-register all services matching a convention
builder.Services.Scan(scan => scan
    .FromAssemblyOf<Program>()
    .AddClasses(classes => classes.AssignableTo<ITransientService>())
    .AsImplementedInterfaces()
    .WithTransientLifetime()
    .AddClasses(classes => classes.AssignableTo<IScopedService>())
    .AsImplementedInterfaces()
    .WithScopedLifetime());

工厂模式

需要在运行时通过逻辑选择实现时使用。

builder.Services.AddScoped<IPaymentProcessor>(sp =>
{
    var config = sp.GetRequiredService<IOptions<PaymentOptions>>().Value;
    return config.Provider switch
    {
        "stripe" => ActivatorUtilities.CreateInstance<StripeProcessor>(sp),
        "paypal" => ActivatorUtilities.CreateInstance<PayPalProcessor>(sp),
        _ => throw new InvalidOperationException($"Unknown payment provider: {config.Provider}")
    };
});

选项注册

// Bind configuration section to a strongly-typed options class
builder.Services.AddOptions<JwtOptions>()
    .BindConfiguration("Jwt")
    .ValidateDataAnnotations()
    .ValidateOnStart();

// Inject as IOptions<T>
public class TokenService(IOptions<JwtOptions> options)
{
    private readonly JwtOptions _jwt = options.Value;
}

反模式

不要在单例中捕获作用域服务

// BAD — DbContext is scoped, captured by singleton = memory leak + stale data
builder.Services.AddSingleton<OrderCache>(); // depends on AppDbContext

// GOOD — use IServiceScopeFactory in singleton
public class OrderCache(IServiceScopeFactory scopeFactory)
{
    public async Task<Order?> GetAsync(Guid id)
    {
        await using var scope = scopeFactory.CreateAsyncScope();
        var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
        return await db.Orders.FindAsync(id);
    }
}

不要将所有服务都注册为单例

// BAD — making a service singleton when it holds mutable state
builder.Services.AddSingleton<OrderService>(); // has DbContext dependency

// GOOD — match the lifetime to the service's needs
builder.Services.AddScoped<OrderService>();

决策指南

场景	建议
无状态服务	作用域（默认）或瞬态
配置 / 缓存	单例
DbContext	作用域（由 AddDbContext 注册）
多个实现	键控服务（策略模式）
横切行为	装饰器模式
按约定注册	Scrutor
运行时选择实现	工厂委托
审查现有注册	get_di_registrations MCP 工具 — 一次调用即可检查生命周期、重复项和依赖捕获风险
强类型配置	AddOptions<T>().BindConfiguration()