利用.NET Core类库System.Reflection.DispatchProxy实现简易Aop

背景

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。

System.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作为拦截器注入
        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;
    }
}

改造成一款简易AOP

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……

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，加入了传入拦截器实例和传入拦截器构造方法参数的功能。

/// <summary>
/// 拦截器接口
/// </summary>
public interface IInterceptor
{
    /// <summary>
    /// 拦截器调用
    /// </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);
}

拦截器要实现这个接口，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">拦截器</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">拦截器类型</param>
    /// <param name="parameters">拦截器构造函数参数值</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:拦截器类型
    /// </summary>
    /// <param name="parameters">拦截器构造函数参数值</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);
    }
}

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);
    }

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