设置一个事件为Null

问题描述：

我有一个code是这样的：

I have a code like this:

public class SomeClass
{
    int _processProgress;
    public int ProcessProgress 
    { 
        get { return _processProgress; } 
        set 
        { 
            _processProgress = value; 
            if (ProcessProgressChanged != null) 
                ProcessProgressChanged(value);
        } 
    }

    public delegate void ProcessProgressChangedEventHandler(int progressPercentage);
    public event ProcessProgressChangedEventHandler ProcessProgressChanged;

    public void ClearProcessProgressChangedEvent()
    {
        this.ProcessProgressChanged = null;
    }
}

请问它的退订的的时候，我称之为 ProcessProgressChanged 事件的所有方法 ClearProcessProgressChangedEvent（）的方法？

Will it unsubscribe all method in the ProcessProgressChanged event when I call the ClearProcessProgressChangedEvent() method?

我的code是在C＃中，框架4，建立在VS2010临，项目是的WinForms。

My code is in C#, framework 4, build in VS2010 Pro, project is in Winforms.

请帮忙。先谢谢了。

答

嗯，这将有效地清除用户的名单，是的（底层委托字段设置为空） - 使下一次 ProcessProgress 时，没有处理程序将被调用。它不是真正的设置的事件到空 - 这是基础字段设置为空。这只是C＃编译器是创建两个事件（一个订阅/退订对方法）和场（存储处理器）使用单一的声明。

Well, it'll effectively clear the list of subscribers, yes (by setting the underlying delegate field to null) - so that the next time ProcessProgress is set, no handlers will be called. It's not really setting the event to null - it's setting the underlying field to null. It's just that the C# compiler is creating both an event (a subscribe/unsubscribe pair of methods) and a field (to store the handlers) using a single declaration.

您可能会发现我的关于事件和委托的有用的文章。

You may find my article about events and delegates useful.

请注意您的活动筹集code目前不是线程安全的。我不知道是否它需要或没有，但你可能想的认为的使用：

Note that your event-raising code currently isn't thread-safe. I don't know whether it needs to be or not, but you might want to consider using:

set 
{ 
    _processProgress = value; 
    var handlers = ProcessProgressChanged;
    if (handlers != null) 
    {
        handlers(value);
    }
}

这样，你不会得到一个的NullReferenceException 如果最后的处理程序是退订的在的的无效检查，但的在调用。

That way you won't get a NullReferenceException if the last handler is unsubscribed after the nullity check but before the invocation.

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