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被订阅时执行某些操作(例如写入控制台)被解雇了?
使用 Reflection 似乎是不可能的,我想尽可能避免使用 Reflection.Emit,因为目前(对我来说)这似乎是唯一的方法。
/EDIT:我不知道事件需要的委托(delegate)签名,这是问题的核心
/EDIT 2:虽然委托(delegate)逆变似乎是个好计划,但我无法做出使用此解决方案所必需的假设
最佳答案
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。
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();
}
}
关于C#动态事件订阅,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/45779/