C# 动态事件订阅 [英] C# Dynamic Event Subscription

问题描述

您将如何动态订阅 C# 事件，以便给定一个对象实例和一个包含事件名称的字符串名称，您订阅该事件并在该事件发生时执行某些操作(例如写入控制台)被解雇了?

How would you dynamically subscribe to a C# event so that given a Object instance and a String name containing the name of the event, you subscribe to that event and do something (write to the console for example) when that event has been fired?

使用 Reflection 这似乎是不可能的，如果可能的话，我想避免使用 Reflection.Emit，因为目前(对我而言)这似乎是唯一的方法.

It would seem using Reflection this isn't possible and I would like to avoid having to use Reflection.Emit if possible, as this currently (to me) seems like the only way of doing it.

/我不知道事件所需的委托签名，这是问题的核心

/ I do not know the signature of the delegate needed for the event, this is the core of the problem

/EDIT 2:虽然委托逆变似乎是一个不错的计划，但我无法做出使用此解决方案所必需的假设

/EDIT 2: Although delegate contravariance seems like a good plan, I can not make the assumption necessary to use this solution

推荐答案

您可以编译表达式树以使用不带任何参数的 void 方法作为任何类型事件的事件处理程序.为了适应其他事件处理程序类型，您必须以某种方式将事件处理程序的参数映射到事件.

You can compile expression trees to use void methods without any arguments as event handlers for events of any type. To accommodate other event handler types, you have to map the event handler's parameters to the events somehow.

 using System;
 using System.Linq;
 using System.Linq.Expressions;
 using System.Reflection;

 class ExampleEventArgs : EventArgs
 {
    public int IntArg {get; set;}
 }

 class EventRaiser
 { 
     public event EventHandler SomethingHappened;
     public event EventHandler<ExampleEventArgs> SomethingHappenedWithArg;

     public void RaiseEvents()
     {
         if (SomethingHappened!=null) SomethingHappened(this, EventArgs.Empty);

         if (SomethingHappenedWithArg!=null) 
         {
            SomethingHappenedWithArg(this, new ExampleEventArgs{IntArg = 5});
         }
     }
 }

 class Handler
 { 
     public void HandleEvent() { Console.WriteLine("Handler.HandleEvent() called.");}
     public void HandleEventWithArg(int arg) { Console.WriteLine("Arg: {0}",arg);    }
 }

 static class EventProxy
 { 
     //void delegates with no parameters
     static public Delegate Create(EventInfo evt, Action d)
     { 
         var handlerType = evt.EventHandlerType;
         var eventParams = handlerType.GetMethod("Invoke").GetParameters();

         //lambda: (object x0, EventArgs x1) => d()
         var parameters = eventParams.Select(p=>Expression.Parameter(p.ParameterType,"x"));
         var body = Expression.Call(Expression.Constant(d),d.GetType().GetMethod("Invoke"));
         var lambda = Expression.Lambda(body,parameters.ToArray());
         return Delegate.CreateDelegate(handlerType, lambda.Compile(), "Invoke", false);
     }

     //void delegate with one parameter
     static public Delegate Create<T>(EventInfo evt, Action<T> d)
     {
         var handlerType = evt.EventHandlerType;
         var eventParams = handlerType.GetMethod("Invoke").GetParameters();

         //lambda: (object x0, ExampleEventArgs x1) => d(x1.IntArg)
         var parameters = eventParams.Select(p=>Expression.Parameter(p.ParameterType,"x")).ToArray();
         var arg    = getArgExpression(parameters[1], typeof(T));
         var body   = Expression.Call(Expression.Constant(d),d.GetType().GetMethod("Invoke"), arg);
         var lambda = Expression.Lambda(body,parameters);
         return Delegate.CreateDelegate(handlerType, lambda.Compile(), "Invoke", false);
     }

     //returns an expression that represents an argument to be passed to the delegate
     static Expression getArgExpression(ParameterExpression eventArgs, Type handlerArgType)
     {
        if (eventArgs.Type==typeof(ExampleEventArgs) && handlerArgType==typeof(int))
        {
           //"x1.IntArg"
           var memberInfo = eventArgs.Type.GetMember("IntArg")[0];
           return Expression.MakeMemberAccess(eventArgs,memberInfo);
        }

        throw new NotSupportedException(eventArgs+"->"+handlerArgType);
     }
 }


 static class Test
 {
     public static void Main()
     { 
        var raiser  = new EventRaiser();
        var handler = new Handler();

        //void delegate with no parameters
        string eventName = "SomethingHappened";
        var eventinfo = raiser.GetType().GetEvent(eventName);
        eventinfo.AddEventHandler(raiser,EventProxy.Create(eventinfo,handler.HandleEvent));

        //void delegate with one parameter
        string eventName2 = "SomethingHappenedWithArg";
        var eventInfo2 = raiser.GetType().GetEvent(eventName2);
        eventInfo2.AddEventHandler(raiser,EventProxy.Create<int>(eventInfo2,handler.HandleEventWithArg));

        //or even just:
        eventinfo.AddEventHandler(raiser,EventProxy.Create(eventinfo,()=>Console.WriteLine("!")));  
        eventInfo2.AddEventHandler(raiser,EventProxy.Create<int>(eventInfo2,i=>Console.WriteLine(i+"!")));

        raiser.RaiseEvents();
     }
 }

这篇关于C# 动态事件订阅的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！