一个多态场般的事件调用 [英] Invocation of a polymorphic field-like event

问题描述

考虑下面的代码：

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???
        }
    }
}

在这段代码的 ReSharper的显示警报多态场类似事件的调用。

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

我的问题是：
是什么实际上意味着？而且它是一个不好的编程习惯警报？
此外，它是一个不好的做法，多态调用事件？ （会意，一个事件只能从已宣布它的类提出。）

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

好吧，你实际上得到的两个的现场喜欢这里的事件。您的覆盖将是压倒一切的添加/删除一部分，但你有两个字段 - 一个在 TableMain ，一个在 TableSub 。只有一个在 TableSub 将永远非空，除非值显式设置在 TableMain ...等等如果 TableMain 曾经试图引发该事件本身，它不会叫同一套处理程序在 TableSub 。基本上，它会出现异常。

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.

正确的做法是提供 TableMain 来让一个受保护的方法由子类引发事件：

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，让事件非虚，并取出在 TableSub 覆盖

请注意您的活动签名不匹配事件的正常惯例 - 任何理由不使用事件处理程序？

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

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