我经常听到/读到以下建议:

在检查事件是否为空并触发它之前,始终要对事件进行复制。这将消除线程的一个潜在问题,即事件在你检查null和你触发事件的位置之间变成null:

// Copy the event delegate before checking/calling
EventHandler copy = TheEvent;

if (copy != null)
    copy(this, EventArgs.Empty); // Call any handlers on the copied list

更新:我认为从阅读优化,这可能也需要事件成员是volatile,但Jon Skeet在他的回答中说,CLR不会优化掉副本。

但与此同时,为了让这个问题发生,另一个线程必须做这样的事情:

// Better delist from event - don't want our handler called from now on:
otherObject.TheEvent -= OnTheEvent;
// Good, now we can be certain that OnTheEvent will not run...

实际的序列可能是这样的:

// Copy the event delegate before checking/calling
EventHandler copy = TheEvent;

// Better delist from event - don't want our handler called from now on:
otherObject.TheEvent -= OnTheEvent;    
// Good, now we can be certain that OnTheEvent will not run...

if (copy != null)
    copy(this, EventArgs.Empty); // Call any handlers on the copied list

重点是OnTheEvent运行后,作者已经取消订阅,但他们只是取消订阅专门避免这种情况发生。当然,真正需要的是在添加和删除访问器中具有适当同步的自定义事件实现。此外,如果在触发事件时持有锁,则可能存在死锁的问题。

So is this Cargo Cult Programming? It seems that way - a lot of people must be taking this step to protect their code from multiple threads, when in reality it seems to me that events require much more care than this before they can be used as part of a multi-threaded design. Consequently, people who are not taking that additional care might as well ignore this advice - it simply isn't an issue for single-threaded programs, and in fact, given the absence of volatile in most online example code, the advice may be having no effect at all.

(在成员声明中分配空委托{}不是更简单吗,这样你就永远不需要在第一时间检查null ?)

Updated: In case it wasn't clear, I did grasp the intention of the advice - to avoid a null reference exception under all circumstances. My point is that this particular null reference exception can only occur if another thread is delisting from the event, and the only reason for doing that is to ensure that no further calls will be received via that event, which clearly is NOT achieved by this technique. You'd be concealing a race condition - it would be better to reveal it! That null exception helps to detect an abuse of your component. If you want your component to be protected from abuse, you could follow the example of WPF - store the thread ID in your constructor and then throw an exception if another thread tries to interact directly with your component. Or else implement a truly thread-safe component (not an easy task).

所以我认为,仅仅做这种复制/检查的习惯是盲目的编程,会给你的代码增加混乱和噪音。要真正防范其他线程,需要做更多的工作。

针对Eric Lippert博客文章的更新:

因此,关于事件处理程序,我错过了一个主要的事情:“事件处理程序必须健壮,即使在事件被取消订阅后也需要被调用”,显然,因此我们只需要关心事件委托为空的可能性。对事件处理程序的需求在任何地方都有记录吗?

所以:“有其他方法可以解决这个问题;例如,初始化处理程序以拥有一个永远不会删除的空操作。但做空检查是标准模式。”

所以我的问题剩下的一个片段是,为什么显式空检查是“标准模式”?另一种方法,分配空委托,只需要将= delegate{}添加到事件声明中,这就消除了每个引发事件的地方的那些臭仪式。可以很容易地确保实例化空委托的成本很低。还是我还遗漏了什么?

当然,就像Jon Skeet所建议的那样,这只是。net 1。X条没有消失的建议,就像它在2005年应该消失的那样?


更新

从c# 6开始,这个问题的答案是:

SomeEvent?.Invoke(this, e);

当前回答

请看这里:http://www.danielfortunov.com/software/%24daniel_fortunovs_adventures_in_software_development/2009/04/23/net_event_invocation_thread_safety 这是正确的解决方案,应该经常使用,而不是所有其他变通方法。

通过使用不做任何事情的匿名方法初始化内部调用列表,可以确保它始终至少有一个成员。因为没有外部方可以引用匿名方法,也没有外部方可以删除该方法,因此委托永远不会为空。” -编程。net组件,第二版,由Juval Löwy

public static event EventHandler<EventArgs> PreInitializedEvent = delegate { };  

public static void OnPreInitializedEvent(EventArgs e)  
{  
    // No check required - event will never be null because  
    // we have subscribed an empty anonymous delegate which  
    // can never be unsubscribed. (But causes some overhead.)  
    PreInitializedEvent(null, e);  
}  

其他回答

我一直在使用这种设计模式来确保事件处理程序在取消订阅后不会执行。到目前为止,它工作得很好,尽管我还没有尝试过任何性能分析。

private readonly object eventMutex = new object();

private event EventHandler _onEvent = null;

public event EventHandler OnEvent
{
  add
  {
    lock(eventMutex)
    {
      _onEvent += value;
    }
  }

  remove
  {
    lock(eventMutex)
    {
      _onEvent -= value;
    }
  }

}

private void HandleEvent(EventArgs args)
{
  lock(eventMutex)
  {
    if (_onEvent != null)
      _onEvent(args);
  }
}

这些天我主要是在Android上使用Mono,当你试图在Activity被发送到后台后更新视图时,Android似乎不喜欢它。

在c# 6及以上版本中,可以使用new ?运算符如下:

TheEvent ?。调用(这,EventArgs.Empty);

这里是MSDN文档。

把你在工地的所有活动都打过去,别插手。Delegate类的设计不可能正确地处理任何其他用法,这一点我将在本文的最后一段解释。

首先,当您的事件处理程序必须已经就是否/如何响应通知做出同步决策时,尝试拦截事件通知是没有意义的。

任何可能被通知的事情,都应该被通知。如果您的事件处理程序正确地处理了通知(例如,它们可以访问权威应用程序状态并只在适当的时候响应),那么随时通知它们并相信它们会正确地响应就可以了。

处理程序不应该收到事件发生通知的唯一时间是事件实际上没有发生的时候!因此,如果你不想让一个处理程序得到通知,停止生成事件(即禁用控件或任何负责检测并在第一时间使事件存在的东西)。

Honestly, I think the Delegate class is unsalvageable. The merger/transition to a MulticastDelegate was a huge mistake, because it effectively changed the (useful) definition of an event from something that happens at a single instant in time, to something that happens over a timespan. Such a change requires a synchronization mechanism that can logically collapse it back into a single instant, but the MulticastDelegate lacks any such mechanism. Synchronization should encompass the entire timespan or instant the event takes place, so that once an application makes the synchronized decision to begin handling an event, it finishes handling it completely (transactionally). With the black box that is the MulticastDelegate/Delegate hybrid class, this is near impossible, so adhere to using a single-subscriber and/or implement your own kind of MulticastDelegate that has a synchronization handle that can be taken out while the handler chain is being used/modified. I'm recommending this, because the alternative would be to implement synchronization/transactional-integrity redundantly in all your handlers, which would be ridiculously/unnecessarily complex.

我看到很多人都倾向于用扩展的方法来做这件事……

public static class Extensions   
{   
  public static void Raise<T>(this EventHandler<T> handler, 
    object sender, T args) where T : EventArgs   
  {   
    if (handler != null) handler(sender, args);   
  }   
}

这为引发事件提供了更好的语法……

MyEvent.Raise( this, new MyEventArgs() );

并且还消除了本地副本,因为它是在方法调用时捕获的。

对于单线程应用程序,你是正确的,这不是一个问题。

但是,如果您正在制作一个公开事件的组件,则无法保证组件的使用者不会采用多线程,在这种情况下,您需要做好最坏的准备。

使用空委托确实解决了这个问题,但也会在每次调用事件时导致性能下降,并可能涉及GC。

您是对的,消费者试图取消订阅才会发生这种情况,但如果他们通过了临时副本,那么就认为消息已经在传输中。

如果你不使用临时变量,也不使用空委托,有人取消订阅,你会得到一个空引用异常,这是致命的,所以我认为代价是值得的。