分配给空委托的事件声明 [英] Event declaration with assignment to empty delegate

问题描述

我对经常在网上偶然发现的一段代码有疑问。例如，

I have question on a piece of code I often stumble across online. For example

  public static event Action<Foo> foo = delegate{  };

我在声明的左侧。但是右手边令人困惑。将事件分配给带有空括号的委托人有什么意义？

I got the left hand side of the declaration. But the right hand side is confusing. What is the point in assigning the event to a delegate with empty parenthesis?

推荐答案

更新：在现代版本的C＃中，推荐使用的模式是将其保留为null并使用<$ c 运算符：

UPDATE: in modern versions of C#, the recommended pattern to use is to leave it as null and use the ?. operator:

event Action<Foo> foo; // leave it as null as the default
// ...
foo?.(new Foo());

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在较旧版本的C＃中，其原因是模式可能是 event 字段的默认值为 null 。这要求您每次想引发事件时都使用null检查-在未初始化的事件 foo（） $ c>字段将导致 NullReferenceException 。只需给它一个合理的默认值（如无操作），便更加方便，这样您就可以直接引发事件，而无需使用包装方法或其他任何方法。

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In older versions of C#, the reason for this pattern could be that the default value of an event field is null. This requires the use of a null-check every-time you want to raise the event - calling foo() on an uninitialised event field would result in a NullReferenceException. It's more convenient to just give it a sensible default value like a no-op, that way you can raise the event directly without having to use a wrapper method or anything.

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