c# – .NET对象事件和处理/ GC

class A : IDisposable
{
   public event EventHandler EventHappened
   {
      add 
      {
         eventTable["EventHappened"] = (EventHandler)eventTable["EventHappened"] + value;
      }
      remove
      {
         eventTable["EventHappened"] = (EventHandler)eventTable["EventHappened"] - value; 
      }
   }

   public void Dispose()
   {
      //Amit: If you have only one event 'EventHappened',//you can clear up the subscribers as follows

      eventTable["EventHappened"] = null;

      //Amit: EventHappened = null will not work here as it is 
      //just a syntactical sugar to clear the compiler generated backing delegate.
      //Since you have added 'add' and 'remove' there is no compiler generated 
      //delegate to clear
      //
      //Above was just to explain the concept.
      //If eventTable is a dictionary of EventHandlers
      //You can simply call 'clear' on it.
      //This will work even if there are more events like EventHappened          
   }
}

class B
{          
   public B(A a)
   {
      a.EventHappened += new EventHandler(this.HandleEventB);

      //You are absolutely right here.
      //class B does not store any reference to A
      //Subscribing an event does not add any reference to publisher
      //Here all you are doing is calling 'Add' method of 'EventHappened'
      //passing it a delegate which holds a reference to B.
      //Hence there is a path from A to B but not reverse.
   }

   public void HandleEventB(object sender,EventArgs args)
   {
   }
}

class C
{          
   public C(A a)
   {
      a.EventHappened += new EventHandler(this.HandleEventC);
   }

   public void HandleEventC(object sender,EventArgs args)
   {
   }
}

class Service
{       
    A _a;

    void Start()
    {
       CustomNotificationSystem.OnEventRaised += new EventHandler(CustomNotificationSystemHandler)

       _a = new A();

       //Amit:You are right all these do not store any reference to _a
       B b1 = new B(_a);
       B b2 = new B(_a);
       C c1 = new C(_a);
       C c2 = new C(_a);
    }

    void CustomNotificationSystemHandler(args)
    {

        //Amit: You decide that _a has lived its life and must be disposed.
        //Here I assume you want to dispose so that it stops firing its events
        //More on this later
        _a.Dispose();

        //Amit: Now _a points to a brand new A and hence previous instance 
        //is eligible for collection since there are no active references to 
        //previous _a now
        _a = new A();
    }    
}

b1,and I know they will now
subscribed to previous instance of _a,and it’s OK by me,BUT in that
example,nobody references the previous instance of _a (b not
holds reference to _a) and by my theory,now
may be collected…or I’m missing something???

正如我在上述代码中的评论所解释的,你没有在这里遗漏任何东西:)

BUT…as I know,it’s not guaranteed when GC will collect the
publisher so in theory,even if subscribers lifetime is greater then
publishers,it can happen that subscriber is legal for collection,but
publisher is still not collected (I don’t know if within closest GC
cycle,GC will be smart enough to collect publisher first and then
subscriber.

由于发布商引用订阅者,所以在发布商之前,订阅者无法收到资格,但是倒序可能是真实的.如果发布者在订阅者之前收集,正如你所说,没有问题.如果订阅者属于比发布商更低的GC代码,则由于发布者持有对订阅者的引用,GC将把订阅者视为可达到并且不会收集.如果两者都属于同一代人,他们将被收集在一起.

since my subscriber do not have direct reference to publisher and
can’t unsubscribe the event,I would like to make publisher to
implement IDisposable

与某些建议相反,如果您确定确定不再需要该对象,我建议您执行处理.简单地更新对象引用可能并不总是导致对象停止发布事件.

请考虑以下代码：

class MainClass
{
    public static Publisher Publisher;

    static void Main()
    {
        Publisher = new Publisher();

        Thread eventThread = new Thread(DoWork);
        eventThread.Start();

        Publisher.StartPublishing(); //Keep on firing events
    }

    static void DoWork()
    {
        var subscriber = new Subscriber();
        subscriber = null; 
        //Subscriber is referenced by publisher's SomeEvent only
        Thread.Sleep(200);
        //We have waited enough,we don't require the Publisher now
        Publisher = null;
        GC.Collect();
        //Even after GC.Collect,publisher is not collected even when we have set Publisher to null
        //This is because 'StartPublishing' method is under execution at this point of time
        //which means it is implicitly reachable from Main Thread's stack (through 'this' pointer)
        //This also means that subscriber remain alive
        //Even when we intended the Publisher to stop publishing,it will keep firing events due to somewhat 'hidden' reference to it from Main Thread!!!!
    }
}

internal class Publisher
{
    public void StartPublishing()
    {
        Thread.Sleep(100);
        InvokeSomeEvent(null);
        Thread.Sleep(100);
        InvokeSomeEvent(null);
        Thread.Sleep(100);
        InvokeSomeEvent(null);
        Thread.Sleep(100);
        InvokeSomeEvent(null);
    }

    public event EventHandler SomeEvent;

    public void InvokeSomeEvent(object e)
    {
        EventHandler handler = SomeEvent;
        if (handler != null)
        {
            handler(this,null);
        }
    }

    ~Publisher()
    {
        Console.WriteLine("I am never Printed");
    }
}

internal class Subscriber
{
    public Subscriber()
    {
        if(MainClass.Publisher != null)
        {
            MainClass.Publisher.SomeEvent += PublisherSomeEvent;
        }
    }

    void PublisherSomeEvent(object sender,EventArgs e)
    {
        if (MainClass.Publisher == null)
        {
            //How can null fire an event!!! Raise Exception
            throw new Exception("Booooooooommmm");
            //But notice 'sender' is not null
        }
    }
}

如果你运行上面的代码,你通常会收到“Booooooooommmm”.所以想法是事件发布者必须停止触发事件,当我们确定它的生命已经到了.

这可以通过Dispose方法完成.

新200的X- 200 X- 200 200 X- 200 200：

>设置一个标志’IsDisposed’,并在触发任何事件之前检查它.
>清除事件订阅者列表(如我在代码中的注释中所建议的).

2的好处是您释放对订户的任何引用,从而实现收集(正如我之前解释的,即使发布者是垃圾,但属于较高的一代,那么它仍然可能延长下一代用户的收集).

不过,诚然,由于发布商“隐藏”的可及性,您将体会到这种表现的行为,这是很罕见的,但是您可以看到2的优点是明确的,并且对于所有的活动发布商,特别是长寿的发行商(Singletons anybody！ ！).这本身使得值得实施Dispose并与2一起去.