【发布时间】：2017-07-05 21:30:00
【问题描述】：
我来自 Ninject，但我决定尝试一下 Autofac，因为它似乎更加活跃。到目前为止，我可以说注册装饰器并不像在 Ninject 中使用 .WhenInjectedExactlyInto 语法那么容易。无论如何，请多多包涵，因为我是 Autofac 新手。
问题来了：
我有类型 A 实现由 A_Decorator 装饰的接口 IA。 A_Decorator 实现了接口IA 和IB，而AB_Decorator 又实现了IA 和IB。 AB_Decorator 接受 IA 和 IB 类型的两个依赖项（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 IA 或 IB 类型的服务时，它们应该引用单个 AB_Decorator 实例。
用词来描述有点棘手，但这是我能想到的最简单的代码示例，它显示了这种情况（我已将实例 ID 和跟踪消息添加到构造函数以查看发生了什么）：
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)，我无法尝试命名注册的各种组合。
有人知道这个问题的解决方案吗？提前致谢。
【问题讨论】：
标签： c# dependency-injection decorator autofac circular-dependency