事件处理方法，提高会议 [英] Event handler raising method convention

问题描述

我只是浏览并在这个问题就来了：

的动作VS委托事件

的从nobug 包含这些代码答案：

 受保护的虚拟无效OnLeave（EmployeeEventArgsË ）{
 VAR处理器=假; 
如果（处理！= NULL）
处理器（这一点，E）; 
} 
  

ReSharper的也使用了创建养法速战速决时，会产生类似的代码

我的问题是，为什么这行必要的：

  VAR处理器=假; 
  

为什么它比写这个？

 受保护的虚拟无效OnLeave（EmployeeEventArgs E）{
如果（离开！= NULL）
离开（这一点，E）; 
} 
  

解决方案

这是更好，因为有一个微小的可能性，即离开成为空检查之后null，但调用（这将导致你的代码抛出之前的NullReferenceException ）。由于委托类型是不可变的，如果你第一次把它分配给一个变​​量这种可能性消失;您的本地副本将不会被转让后的任何修改离开受到影响。

请注意，虽然这种做法也造成相反的一个问题;这意味着有一个（微小的，但现有的）可能性的事件处理程序被调用的它已被从事件的拆卸之后。这种情形当然要处理的优雅以及。

I was just browsing and came across this question:

Action vs delegate event

The answer from nobug included this code:

protected virtual void OnLeave(EmployeeEventArgs e) {
  var handler = Leave;
  if (handler != null)
    handler(this, e);
}

Resharper also generates similar code when using the "create raising method" quick-fix.

My question is, why is this line necessary?:

var handler = Leave;

Why is it better than writing this?:

protected virtual void OnLeave(EmployeeEventArgs e) {
  if (Leave != null)
    Leave(this, e);
}

解决方案

It's better because there is a tiny possibility that Leave becomes null after the null check, but before the invocation (which would cause your code to throw a NullReferenceException). Since the delegate type is immutable, if you first assign it to a variable this possibility goes away; your local copy will not be affected by any changes to Leave after the assignment.

Note though that this approach also creates a issue in reverse; it means that there is a (tiny, but existing) possibility that an event handler gets invoked after it has been detached from the event. This scenario should of course be handled gracefully as well.

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