调用多态字段类事件 [英] Invocation of a polymorphic field-like event

问题描述

考虑以下代码：

public class TableMain {
    public virtual event Action UpdateFilter;
    ....
}

public class TableSub : TableMain {
    public override event Action UpdateFilter;

    public void UpdateQuery() {
        .....
        if (UpdateFilter!=null) {
              UpdateFilter(); // Invocation of polymorphic field-like event???
        }
    }
}

在此代码 ReSharper 显示警报调用多态字段类事件。

In this code ReSharper shows the alert "invocation of polymorphic field-like event".

我的问题是：
它实际上是什么意思？这是一个不好的编程实践的警戒吗？
另外，这是一个不好的做法来调用一个事件的多态吗？ （知道一个事件只能从已经声明的类中提出）。

My question is: What does it actually mean? And is it an alert for a bad programming practice? Also, is it a bad practice to call an event polymorphically? (Knowing that an event can only be raised from the class which has declared it.)

推荐答案

嗯，你实际上已经这两个类似场景的事件。您的覆盖将覆盖添加/删除部分，但您将有两个字段 - 一个位于 TableMain 中，一个位于 TableSub 。只有 TableSub 中的一个将永远是非空的，除非该值在 TableMain ...中明确设置如果 TableMain 尝试提升事件本身，则不会像 TableSub 中那样调用相同的处理程序集。基本上，它会表现得奇怪。

Well, you've actually got two field-like events here. Your override will be overriding the add/remove part, but you'll have two fields - one in TableMain and one in TableSub. Only the one in TableSub will ever be non-null, unless the value is set explicitly in TableMain... so if TableMain ever tries to raise the event itself, it won't call the same set of handlers as in TableSub. Basically, it's going to behave strangely.

正确的方法是在 TableMain 中提供一个受保护的方法，以允许事件由子类引发：

The right approach is to provide a protected method in TableMain to allow the event to be raised by subclasses:

protected void OnUpdateFilter()
{
    Action handler = UpdateFilter;
    if (handler != null)
    {
        handler();
    }
}

然后使事件非虚拟，并删除覆盖 TableSub 。

Then make the event non-virtual, and remove the override in TableSub.

请注意，事件签名与事件的常规约定不符 - 不使用 EventHandler ？

Note that your event signature doesn't match the normal convention for events - any reason for not using EventHandler?

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