.NET Core类库System.Reflection.DispatchProxy实现简易Aop的方法

前言

aop即是面向切面编程，众多Aop框架里Castle是最为人所知的，另外还有死去的Spring.NET，当然，.NET Core社区新秀AspectCore在性能与功能上都非常优秀，已经逐渐被社区推崇和有越来越多的人使用。感谢柠檬同学的礼物！

如果大家出于自身需求或者学习，想实现一个Aop，是不是觉得一来就要使用Emit去做？最近我了解到了System.Reflection.DispatchProxy这个corefx类库，已经实现了动态代理功能。

1|1System.Reflection.DispatchProxy

下面演示一下它的使用方法：

class Program
{
 static void Main(string[] args)
 {
 //创建代理类，并把SampleProxy作为注入
 var sampleProxy = (targetInterface)SampleProxy.Create<targetInterface, SampleProxy>();
 //执行接口方法
 sampleProxy.Write("here is invoke by proxy");
 }
}

//需要被生成代理实例的接口
public interface targetInterface
{
 //这个方法会被代理类实现
 void Write(string writesomeshing);
}

public class SampleProxy : DispatchProxy
{
 /// <summary>
 /// 拦截调用
 /// </summary>
 /// <param name="method">所拦截的方法信息</param>
 /// <param name="parameters">所拦截方法被传入的参数指</param>
 /// <returns></returns>
 protected override object Invoke(MethodInfo targetMethod, object[] args)
 {
 Console.WriteLine(args[0]);
 return null;
 }
}

2|0改造成一款简易AOP

2|1为什么？

System.Reflection.DispatchProxy只有一个Api，就是objecct Create<T,TProxy>() where TProxy:DispatchProxy,约束了只能传入泛型参数，并不能从方法传入类型，这就会带来很多问题。而更可气的是，给官方提了issue之后，还是不给增加这个api……

2|2改造方法

幸好，在那个issue下，issue作者提供了一个解决方案，就是用反射来构造这个泛型方法。我还在这基础上，封装了一下，加入了传入实例和传入构造方法参数的功能。

/// <summary>
/// 接口
/// </summary>
public interface IInterceptor
{
 /// <summary>
 /// 调用
 /// </summary>
 /// <param name="target">代理实例</param>
 /// <param name="method">所拦截的方法</param>
 /// <param name="parameters">所拦截方法传入的参数值</param>
 /// <returns>返回值会传递给方法返回值</returns> 
 object Intercept(object target, MethodInfo method, object[] parameters);
}

要实现这个接口，下面是对DispatchProxy的封装，实现更多创建代理实例的方法

public class ProxyGenerator : DispatchProxy
{
 private IInterceptor interceptor { get; set; }

 /// <summary>
 /// 创建代理实例
 /// </summary>
 /// <param name="targetType">所要代理的接口类型</param>
 /// <param name="interceptor"></param>
 /// <returns>代理实例</returns>
 public static object Create(Type targetType, IInterceptor interceptor)
 {
 object proxy = GetProxy(targetType);
 ((ProxyGenerator)proxy).CreateInstance(interceptor);
 return proxy;
 }

 /// <summary>
 /// 创建代理实例
 /// </summary>
 /// <param name="targetType">所要代理的接口类型</param>
 /// <param name="interceptorType">类型</param>
 /// <param name="parameters">构造函数参数值</param>
 /// <returns>代理实例</returns>
 public static object Create(Type targetType, Type interceptorType, params object[] parameters)
 {
 object proxy = GetProxy(targetType);
 ((ProxyGenerator)proxy).CreateInstance(interceptorType, parameters);
 return proxy;
 }


 /// <summary>
 /// 创建代理实例 TTarget:所要代理的接口类型 TInterceptor:类型
 /// </summary>
 /// <param name="parameters">构造函数参数值</param>
 /// <returns>代理实例</returns>
 public static TTarget Create<TTarget, TInterceptor>(params object[] parameters) where TInterceptor : IInterceptor
 {
 var proxy = GetProxy(typeof(TTarget));
 ((ProxyGenerator)proxy).CreateInstance(typeof(TInterceptor), parameters);
 return (TTarget)proxy;
 }

 private static object GetProxy(Type targetType)
 {
 var callexp = Expression.Call(typeof(DispatchProxy), nameof(DispatchProxy.Create), new[] { targetType, typeof(ProxyGenerator) });
 return Expression.Lambda<Func<object>>(callexp).Compile()();
 }

 private void CreateInstance(Type interceptorType, object[] parameters)
 {
 var ctorParams = parameters.Select(x => x.GetType()).ToArray();
 var paramsExp = parameters.Select(x => Expression.Constant(x));
 var newExp = Expression.New(interceptorType.GetConstructor(ctorParams), paramsExp);
 this.interceptor = Expression.Lambda<Func<IInterceptor>>(newExp).Compile()();
 }

 private void CreateInstance(IInterceptor interceptor)
 {
 this.interceptor = interceptor;
 }

 protected override object Invoke(MethodInfo method, object[] parameters)
 {
 return this.interceptor.Intercept(method, parameters);
 }
}

2|3使用方法

 class Program
 {
 static void Main(string[] args)
 {
 var poxy1 = (targetInterface)ProxyGenerator.Create(typeof(targetInterface), new SampleProxy("coreproxy1"));
 poxy1.Write("here was invoked"); //---> "here was invoked by coreproxy1"

 var poxy2 = (targetInterface)ProxyGenerator.Create(typeof(targetInterface), typeof(SampleProxy), "coreproxy2");
 poxy2.Write("here was invoked"); //---> "here was invoked by coreproxy2"

 var poxy3 = ProxyGenerator.Create<targetInterface, SampleProxy>("coreproxy3");
 poxy3.Write("here was invoked"); //---> "here was invoked by coreproxy3"
 }
 }


 public class SampleProxy : IInterceptor
 {
 private string proxyName { get; }

 public SampleProxy(string name)
 {
 this.proxyName = name;
 }

 public object Intercept(MethodInfo method, object[] parameters)
 {
 Console.WriteLine(parameters[0] + " by " + proxyName);
 return null;
 }
 }

 public interface targetInterface
 {
 void Write(string writesome);
 }

3|0总结

总结一下就是，微软爸爸给我们的这个轮子还是即轻便又很好用的。

本文的实例代码可以在我的github上找到：https://github.com/ElderJames/CoreProxy

好了，