Question

我试图更好地了解事件及其处理程序的工作原理，但我不明白为什么在举办活动时通常会优先提出相同的事件，即我们的事件本身。更具体地说，在查看msdn doc（https://msdn.microsoft.com/en-us/library/db0etb8x.aspx）时，它看起来像是：

class Counter
{
    private int threshold;
    private int total;

    public Counter(int passedThreshold)
    {
        threshold = passedThreshold;
    }

    public void Add(int x)
    {
        total += x;
        if (total >= threshold)
        {
            ThresholdReachedEventArgs args = new ThresholdReachedEventArgs();
            args.Threshold = threshold;
            args.TimeReached = DateTime.Now;
            OnThresholdReached(args);
        }
    }

    protected virtual void OnThresholdReached(ThresholdReachedEventArgs e)
    {
        EventHandler<ThresholdReachedEventArgs> handler = ThresholdReached;
        if (handler != null)
        {
            handler(this, e);
        }
    }

    public event EventHandler<ThresholdReachedEventArgs> ThresholdReached;
}

我不明白为什么＆＃34;处理程序＆＃34;是在OnThresholdReached函数中创建的，而不是

protected virtual void OnThresholdReached(ThresholdReachedEventArgs e)
    {
        if (ThresholdReached!= null)
        {
            ThresholdReached(this, e);
        }
    }

我们为什么要创建这个＆＃34;处理程序＆＃34; ？

Answer 1

考虑以下代码：

    if (ThresholdReached!= null)
    {
        ThresholdReached(this, e);
    }

如果在ThresholdReached之后但在调用if (ThresholdReached!= null)之前删除ThresholdReached(this, e);的处理程序，多线程代码会发生什么？

获取处理程序的副本可以防止出现这种情况，并使事件的引发成为线程安全的。

EventHandler正确地提升事件

1 个答案: