多接口装饰器——Autofac 中的循环依赖之谜

Question

我来自 Ninject，但我决定尝试一下 Autofac，因为它的开发似乎更加活跃。到目前为止，我可以说注册装饰器并不像使用 .WhenInjectedExactlyInto 语法的 Ninject 中那么容易。无论如何，请耐心等待，因为我是 Autofac 新手。

这是问题所在：

我有类型 A 实现由 A_Decorator 修饰的接口 IA。 A_Decorator 实现了接口 IA 和 IB，反过来应该由 AB_Decorator 装饰，它也实现了 IA 和 IB。 AB_Decorator 接受类型为 IA 和 IB 的两个依赖项（因此它是两者的装饰器）但它们都应该解析为 A_Decorator 的同一个实例。它看起来像这样：AB_Decorator(A_Decorator(A) as IA, A_Decorator(A) as IB)。当从 Autofac 容器请求类型 IA 或类型 IB 的服务时，它们应该引用单个 AB_Decorator 实例。

用文字描述有点棘手，但这是我能想到的最简单的代码示例，它显示了这种情况（我已经向构造函数添加了实例 ID 和跟踪消息以查看发生了什么）：

using System;
using Autofac;

namespace AutofacExample
{
    internal interface IA { }

    internal interface IB { }

    class A : IA
    {
        static int _instanceCounter;
        readonly int Id = ++_instanceCounter;

        public A()
        {
            Console.WriteLine(this);
        }

        public override string ToString()
        {
            return $"{GetType().Name}[{nameof(Id)}={Id}]";
        }
    }

    class A_Decorator : IA, IB
    {
        static int _instanceCounter = 10;
        readonly int Id = ++_instanceCounter;

        /* decorated1 should reference instance of A */

        public A_Decorator(IA decoratedA)
        {
            Console.WriteLine($"{this}({nameof(decoratedA)}={decoratedA})");
        }

        public override string ToString()
        {
            return $"{GetType().Name}[{nameof(Id)}={Id}]";
        }
    }

    class AB_Decorator : IA, IB
    {
        static int _instanceCounter = 100;
        readonly int Id = ++_instanceCounter;

        /* Both decorated1 and decorated2 should reference the same instance of A_Decorator */

        public AB_Decorator(IA decoratedA, IB decoratedB)
        {
            Console.WriteLine($"{this}({nameof(decoratedA)}={decoratedA}, {nameof(decoratedB)}={decoratedB})");
        }

        public override string ToString()
        {
            return $"{GetType().Name}[{nameof(Id)}={Id}]";
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            ContainerBuilder builder = new ContainerBuilder();

            builder
                .RegisterType<A>()
                .Named<IA>(nameof(A))
                .SingleInstance();

            builder
                .RegisterType<A_Decorator>()
                .Named<IA>(nameof(A_Decorator))
                .Named<IB>(nameof(A_Decorator))
                .SingleInstance();

            builder
                .RegisterType<AB_Decorator>()
                .Named<IA>(nameof(AB_Decorator))
                .Named<IB>(nameof(AB_Decorator))
                .SingleInstance();

            /* A is decorated by A_Decorator as IA */
            builder
                .RegisterDecorator<IA>(
                    (c, decorated) =>
                        c.ResolveNamed<IA>(nameof(A_Decorator), TypedParameter.From(decorated)),
                    nameof(A))
                //.Keyed<IA>("innerA")
                //.Keyed<IB>("innerB")
                .SingleInstance();

            /* Trying to register AB_Decorator as IA creates circular dependency */
            //builder
            //    .RegisterDecorator<IA>(
            //        (c, decorated) =>
            //            c.ResolveNamed<IA>(nameof(AB_Decorator), TypedParameter.From(decorated)),
            //        "innerA")
            //    .SingleInstance();

            /* A_Decorator is decorated by AB_Decorator as IB */
            builder
                .RegisterDecorator<IB>(
                        (c, decorated) =>
                            c.ResolveNamed<IB>(nameof(AB_Decorator), TypedParameter.From(decorated)),
                        nameof(A_Decorator) /* "innerB" */)
                    .SingleInstance();

            IContainer container = builder.Build();

            IA a = container.Resolve<IA>();
            IB b = container.Resolve<IB>();

            Console.WriteLine($"{nameof(a)} == {nameof(b)} ? {ReferenceEquals(a, b)}");
            Console.WriteLine($"{nameof(a)} is {a.GetType().Name}");
            Console.WriteLine($"{nameof(b)} is {b.GetType().Name}");
        }
    }
}

不幸的是请求 IA 的实例给我 A_Decorator，而对于 IB 我得到 AB_Decorator。尝试取消注释额外的装饰器注册块会导致循环依赖异常（DependencyResolutionException: Circular component dependency detected: System.Object -> AutofacExample.AB_Decorator -> System.Object -> AutofacExample.AB_Decorator），我无法尝试命名注册的各种组合。

有人知道解决这个问题的方法吗？提前致谢。

Answer 1

问题

问题在于 AB_Decorator 的装饰器注册。特别是解析 AB_Decorator:

的 lambda 函数

( c, decorated ) => c.ResolveNamed<IA>( nameof( AB_Decorator ), TypedParameter.From( decorated ) );

AB_Decorator 的构造函数接受 2 个参数，它们应该是 A_Decorator 的同一个实例，它作为 decorated 提供给 lambda。但是，decorated 仅作为参数通过 TypedParameter.From( decorated ) 传递一次。因此 Autofac 将尝试通过容器解析第二个参数。

现在 IB 的注册表明我们应该得到一个 A_Decorator 包含在 AB_Decorator 中的单例实例。所以要解析IB，容器必须构造AB_Decorator。问题来了，我们目前正在尝试将 AB_Decorator 解析为 IA，但我们需要一个 IB 来满足为 [=26] 构建的 AB_Decorator 的构造函数参数=]. IB 在容器中注册为 AB_Decorator。所以你得到：

AB_Decorator(A_Decorator(A) as IA, AB_Decorator(A_Decorator(A) as IA, AB_Decorator(etc...))

解决方案

我们需要在解析AB_Decorator时将decorated传入两个参数。像这样：

builder
    .RegisterDecorator<IA>(
        ( c, decorated ) =>

            c.ResolveNamed<IA>( nameof( AB_Decorator ),
                new TypedParameter( typeof( IA ), decorated ),
                new TypedParameter( typeof( IB ), decorated )
            )
        ,"innerA"
    )
    .SingleInstance();


builder
    .RegisterDecorator<IB>(
        ( c, decorated ) =>

            c.ResolveNamed<IB>( nameof( AB_Decorator ),
                new TypedParameter( typeof( IA ), decorated ),
                new TypedParameter( typeof( IB ), decorated )
            )
        , nameof( A_Decorator ) /* "innerB" */
    )
    .SingleInstance();

现在我们向 IA 和 IB 参数发送 decorated，即 A_Decorator。直接构造 TypedParameter 实例允许我在参数列表中指定我希望实例实现的类型，在本例中为 AB_Decorator。

Answer 2

给你：

ContainerBuilder builder = new ContainerBuilder();

builder
    .RegisterType<A>()
    .Named<IA>(nameof(A))
    .SingleInstance();

builder
    .RegisterType<A_Decorator>()
    .Named<IA>(nameof(A_Decorator))
    .Named<IB>(nameof(A_Decorator))
    .WithParameter(new ResolvedParameter((pi, c) => pi.Name == "decoratedA", 
        (pi, c) => c.ResolveNamed<IA>(nameof(A))))
    .SingleInstance();

builder
    .RegisterType<AB_Decorator>()
    .As<IA, IB>()
    .WithParameter(new ResolvedParameter((pi, c) => pi.Name == "decoratedA",
        (pi, c) => c.ResolveNamed<IA>(nameof(A_Decorator))))
    .WithParameter(new ResolvedParameter((pi, c) => pi.Name == "decoratedB",
        (pi, c) => c.ResolveNamed<IB>(nameof(A_Decorator))))
    .SingleInstance();

IContainer container = builder.Build();

打印：

A[Id=1]
A_Decorator[Id=11](decoratedA=A[Id=1])
AB_Decorator[Id=101](decoratedA=A_Decorator[Id=11], decoratedB=A_Decorator[Id=11])
a == b ? True
a is AB_Decorator
b is AB_Decorator

API 比较混乱，因为在这种情况下你不需要RegisterDecorator()（它是为了一次装饰一整套组件）。

(要是能整条烤就好了:

    .WithParameter(new ResolvedParameter((pi, c) => pi.Name == "decoratedA", 
        (pi, c) => c.ResolveNamed<IA>(nameof(A))))

成语在 Autofac 中简化为 WithParameter() 重载；如果您在这里看到胜利，我认为这是在项目的问题跟踪器上提出的一个很好的建议。）

多接口装饰器——Autofac 中的循环依赖之谜

Decorator for multiple interfaces - a circular dependency riddle in Autofac

c#

dependency-injection

circular-dependency

decorator

autofac

问题

解决方案