多个接口的装饰器- Autofac中的循环依赖之谜

Question

我来自于Ninject，但我决定尝试一下Autofac，因为它似乎开发得更活跃。到目前为止，我可以说注册装饰器并不像使用.WhenInjectedExactlyInto语法的Ninject那么简单。无论如何，请容忍我，因为我是一个Autofac新手。

以下是问题所在：

我有实现由A_Decorator修饰的接口IA的A类型。A_Decorator实现了IA和IB接口，反过来又应该由同时实现IA和IB的AB_Decorator来装饰。AB_Decorator接受类型IA和IB的两个依赖项(因此它是这两个类型的装饰器)，但它们都应该解析为同一个A_Decorator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IA类型或IB类型的服务时，它们应该引用单个AB_Decorator实例。

用word来描述有点棘手，但下面是我能想到的最简单的代码示例，它说明了这种情况(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的实例给了我A_Decorator，而对于IB，我得到了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)，并且我无法尝试命名注册的各种组合。

有没有人知道解决这个问题的办法？提前谢谢。

Answer 1

问题

问题出在AB_Decorator的装饰器注册上。具体地说就是解析AB_Decorator的lambda函数

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

AB_Decorator的构造函数接受两个参数，这两个参数应该是作为decorated提供给lambda的A_Decorator的相同实例。但是，通过TypedParameter.From( decorated )仅将decorated作为参数传递一次。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，我们应该获得一个包装在AB_Decorator中的带有A_Decorator的单一实例。因此，要解析IB，容器必须构造AB_Decorator。这就是问题所在，我们目前正在尝试将AB_Decorator解析为IA，但是我们需要一个IB来满足为IA构造的AB_Decorator的构造函数参数。并且IB在容器中注册为AB_Decorator。所以你会得到：

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

解决方案

在解析AB_Decorator时，我们需要将decorated传入这两个参数。如下所示：

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，它是A_Decorator。直接构造TypedParameter实例允许我在参数列表中指定希望实例满足的类型，在本例中为AB_Decorator。

Answer 2

这就是了：

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

这个应用程序接口令人困惑，因为在这个场景中您不需要RegisterDecorator() (它用于一次装饰一整套组件)。

(如果我们可以烘焙整个：

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

在Autofac中使用更简单的WithParameter()重载；如果你在这里看到了胜利，我认为这将是一个很好的建议来提出项目的问题跟踪。)