模式实施INotifyPropertyChanged的？ [英] Pattern for implementing INotifyPropertyChanged?

问题描述

我已经看到用于实现INotifyPropertyChanged的以下模式

I have seen the following pattern used for implementing INotifyPropertyChanged

private void NotifyPropertyChanged(string propertyName)
{
    PropertyChangedEventHandler handler = PropertyChanged;
    if (handler != null)
    {
        handler(this, new PropertyChangedEventArgs(propertyName));
    }
}

public event PropertyChangedEventHandler PropertyChanged;

有人能向我解释，在 VAR处理的必要性，检查它是否为null与直接检查之前=的PropertyChanged 分配的PropertyChanged == NULL 直接？

感谢

推荐答案

埃里克利珀解释了这本博客文章详细介绍：的Events和种族的。

Eric Lippert explains this in details in this blog article: Events and races.

基本上，这个想法是要避免出现竞争状况的情况下，另一个线程退订的最后一个处理此事件您检查后的PropertyChanged！= NULL ，但在此之前实际调用的PropertyChanged 。如果您在处理程序的本地副本，这是不可能发生的（但你可能最终会调用一个刚刚退订处理程序）

Basically, the idea is to avoid a race condition in case another thread unsubscribes the last handler for this event after you check PropertyChanged != null, but before you actually invoke PropertyChanged. If you make a local copy of the handler, this cannot happen (but you might end up calling a handler that's just been unsubscribed)

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