C＃事件实现（文章vs反射器） [英] The C# events implementation (articles vs reflector)

问题描述

public class EventsType
{
public event EventHandler> NewEvent;

  public void SmthHappened（string数据）
 {
 MyEventArgs< Object> eventArgs = new MyEventArgs< Object>（data）; 
 OnNewEvent（eventArgs）; 
} 
 
 private void OnNewEvent（MyEventArgs< Object> eventArgs）
 {
 EventHandler< MyEventArgs< Object>> tempEvent = NewEvent; 
 
 if（tempEvent！= null）
 {
 tempEvent（this，eventArgs）; 
} 
} 
} 
  

我预计C＃将如下翻译NewEvent：

  private EventHandler< MyEventArgs< object>> _newEvent; 
 
 public event EventHandler< MyEventArgs< object>> NewEvent 
 {
 [MethodImpl（MethodImplOptions.Synchronized）] 
添加
 {
 _newEvent =（EventHandler< MyEventArgs< object>）Delegate.Combine（_newEvent，值）; 
} 
 [MethodImpl（MethodImplOptions.Synchronized）] 
删除
 {
 _newEvent =（EventHandler< MyEventArgs< object>）Delegate.Remove（_newEvent，value ）; 
} 
} 
  

，但反射器说这是以这种方式实现的：

  public event EventHandler< MyEventArgs< object>> NewEvent 
 {
 add 
 {
 EventHandler< MyEventArgs< object>> handler2; 
 EventHandler< MyEventArgs< object>> newEvent = this.NewEvent; 
 do 
 {
 handler2 = newEvent; 
 EventHandler< MyEventArgs< object>> handler3 =（EventHandler< MyEventArgs< object>）Delegate.Combine（handler2，value）; 
 newEvent = Interlocked.CompareExchange< EventHandler< MyEventArgs< object>>>（ref this.NewEvent，handler3，handler2）; 
} 
 while（newEvent！= handler2）; 
} 
删除
 {
 EventHandler< MyEventArgs< object>> handler2; 
 EventHandler< MyEventArgs< object>> newEvent = this.NewEvent; 
 do 
 {
 handler2 = newEvent; 
 EventHandler< MyEventArgs< object>> handler3 =（EventHandler< MyEventArgs< object>>）Delegate.Remove（handler2，value）; 
 newEvent = Interlocked.CompareExchange< EventHandler< MyEventArgs< object>>>（ref this.NewEvent，handler3，handler2）; 
} 
 while（newEvent！= handler2）; 
} 
} 
  

请问smb解释一下为什么会这样？

解决方案

Yup： C＃4在这方面做了一些改变。它使得线程安全无锁定。这不是唯一的改变 - 它也改变了类中类场景事件的引用如何解决：+ =和 - =现在通过添加和删除位，而不是直接使用后备字段。 p>

请注意，此更改会影响使用C＃4编译器编译的代码，即使对较旧的框架;因为它使用了一种新的方法（ Monitor.TryEnter（object，out bool） ）。

public class EventsType { public event EventHandler> NewEvent;

    public void SmthHappened(string data)
    {
        MyEventArgs<Object> eventArgs = new MyEventArgs<Object>(data);
        OnNewEvent(eventArgs);
    }

    private void OnNewEvent(MyEventArgs<Object> eventArgs)
    {
        EventHandler<MyEventArgs<Object>> tempEvent = NewEvent;

        if (tempEvent != null)
        {                
            tempEvent(this, eventArgs);
        }
    }
}

I expected that the C# compiler will translate the NewEvent like this:

private EventHandler<MyEventArgs<object>> _newEvent;

public event EventHandler<MyEventArgs<object>> NewEvent
{
    [MethodImpl(MethodImplOptions.Synchronized)]
    add
    {
    _newEvent = (EventHandler<MyEventArgs<object>>)Delegate.Combine(_newEvent, value);
    }
    [MethodImpl(MethodImplOptions.Synchronized)]
    remove
    {
    _newEvent = (EventHandler<MyEventArgs<object>>)Delegate.Remove(_newEvent, value);
    }
}

, but the Reflector says that it is implemented this way:

public event EventHandler<MyEventArgs<object>> NewEvent
{
    add
    {
        EventHandler<MyEventArgs<object>> handler2;
        EventHandler<MyEventArgs<object>> newEvent = this.NewEvent;
        do
        {
            handler2 = newEvent;
            EventHandler<MyEventArgs<object>> handler3 = (EventHandler<MyEventArgs<object>>) Delegate.Combine(handler2, value);
            newEvent = Interlocked.CompareExchange<EventHandler<MyEventArgs<object>>>(ref this.NewEvent, handler3, handler2);
        }
        while (newEvent != handler2);
    }
    remove
    {
        EventHandler<MyEventArgs<object>> handler2;
        EventHandler<MyEventArgs<object>> newEvent = this.NewEvent;
        do
        {
            handler2 = newEvent;
            EventHandler<MyEventArgs<object>> handler3 = (EventHandler<MyEventArgs<object>>) Delegate.Remove(handler2, value);
            newEvent = Interlocked.CompareExchange<EventHandler<MyEventArgs<object>>>(ref this.NewEvent, handler3, handler2);
        }
        while (newEvent != handler2);
    }
}

Please, smb explain me why is it so?

解决方案

Yup: C# 4 has made some changes in this area, basically. It makes it thread-safe without locking. That's not the only change - it also changes how references to field-like events within the class are resolved: += and -= now go through the "add" and "remove" bits rather than working directly with the backing field.

Note that this change affects code compiled with the C# 4 compiler even against older frameworks; there are also changes to locking which only affect code compiled against .NET 4, as it uses a new method (Monitor.TryEnter(object, out bool)).

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