Microsoft Patterns & Practices团队在2007年发布的Enterprise Library 3.0 February 2007 CTP中，我们惊喜地发现了AOP的踪迹，其名为Policy Injection Application Block（PIAB）。Enterprise Library的产品经理Tom Hollander和架构师Edward Jezierski都相继在自己的博客上介绍了PIAB。从特征与功能来看，已经基本具备了AOP的要求。
从技术实现来看，PIAB并没有特别的创新，沿用了大多数致力于.NET平台AOP研究人员的思路，采用了Remoting Proxy技术。PIAB定义了特殊的工厂类，通过它添加Policy，然后创建代理对象实例。受到Remoting Proxy技术的限制，所谓的代理对象必须继承自MashalByRefObject。例如：
public class LoggingTarget : MarshalByRefObject
{
public string DoSomething(int one, string two, double three)
{
return string.Format("{1}: {0} {2}", one, two, three);
}
}
添加Policy与创建Aspect对象的方法如下：
private PolicySet GetLoggingPolicies()
{
PolicySet policies = new PolicySet();
Policy simpleInputsPolicy = new Policy("simpleInputsPolicy");
simpleInputsPolicy.RuleSet.Add(new MatchByNameRule("MethodWithSimpleInputs"));
simpleInputsPolicy.Handlers.Add(new SignatureCheckingHandler(new Type[] { typeof(int), typeof(string) }));
policies.Add(simpleInputsPolicy);
return policies;
}
private LoggingTarget CreateTarget()
{
InterceptionFactory factory = new InterceptionFactory();
factory.AddPolicies(GetLoggingPolicies());
return factory.CreateNew<LoggingTarget>();
}
代理对象（Proxy Object）会通过Handler链定位到真实对象（Real Object），而Policy则被注入到代理对象和真实对象中。整个流程如图：
在Policy中，包含了一个Matching Rules集合以及Handlers Pipeline。从AOP技术的角度来看，代理对象所指代的真实对象就是“Aspect”，而Policy则是切入点，它可以通过Matching Rules来定位代理对象的方法，至于Handler则近似于Advise。
Authorization横切关注点（Cross-Cutting Concerns）的处理方式如图所示：
PIAB目前预定义的Handler包括Validation Handler、Logging Handler、Exception Handling Handler、Authorization Handler、Caching Handler。这些Handler与Enterprise Library中的其他Application Block几乎是一一对应的。事实上，权限认证、日志、异常处理、缓存等，恰恰都是AOP技术最重要的关注点。在February 2007 CTP版本之前的Application Block，实际上已经具备了AOP的雏形。然后，由于它在“横切”与“注入”方面的缺乏，始终无法达到AOP所要求的重用目的。Policy Injection Application Block正好弥补了这样的缺憾。
本人也曾利用Remoting Proxy技术开发了.NET平台下的AOP组件，基本实现了AOP的主要功能。然后因为两个困惑，使得这一工作没有继续进行下去。
1、使Aspect对象继承自MarshalByRefObject是否过于专制？鉴于许多语言都具备单继承的特性，如果剥夺了一个类继承的能力，就使得程序的设计者多了很多约束。这是否是明智的选择呢？
2、继承自MarshalByRefObject后，效率究竟如何？我曾经做个这样的测试，发现继承自MarshalByRefObject类的对象比没有继承MarshalByRefObject类的对象，在性能上相差了几十倍。PIAB采用Remoting Proxy技术，是否在性能上有所考虑呢？
>>Source Link
>>Blog: joycode
>>Publish Date: 3/21/2007 1:00:26 PM
>>Keywords: policy handler
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