事件处理程序和空条件运算符 [英] Event handler and null-conditional operator

问题描述

例如，实现 INotifyPropertyChanged 接口:

 公共事件PropertyChangedEventHandler PropertyChanged;私人无效OnPropertyChanged([CallerMemberName]字符串propertyName = null){var handler = PropertyChanged;如果(处理程序！= null)handler.Invoke(this，new PropertyChangedEventArgs(propertyName));} 

两件事:

1. 将事件复制到本地变量以防止多线程错误(
   2. 检查它是否为null，以防止 NullReferenceException

   但是现在，我们可以使用?.运算符进行null检查.如果我使用它，Resharper处于闲置状态:

   因此，问题是:如果我使用空条件运算符，是否应该将事件 ProperyChanged 复制到局部变量?

   解决方案

   如果我使用空条件运算符，应该将事件ProperyChanged复制到本地变量吗?

   不，没有必要.实际上，引入空条件运算符的主要原因之一是使用此模式简化了代码.它与将源值复制到局部变量具有相同的效果，并从本质上避免了复制到局部变量"技术旨在解决的检查并使用"并发陷阱.

   查看相关文章:
   调用事件，h(args)与EventName?.Invoke()(几乎完全是重复的…它确实从一个略微的角度解决了这个问题虽然有不同的角度)
   为什么在调用自定义事件之前应该检查null?
   使用扩展方法引发C#事件-不好吗?
   在引发事件之前是否有任何理由将事件分配给局部变量?

   For example, implement INotifyPropertyChanged interface:

   public event PropertyChangedEventHandler PropertyChanged;
   
   private void OnPropertyChanged([CallerMemberName] string propertyName = null)
   {
       var handler = PropertyChanged;
       if (handler != null)
           handler.Invoke(this, new PropertyChangedEventArgs(propertyName));
   }
   

   Two things:

   1. Copy event to local variable to prevent errors with multithreading (here are some examples). Resharper gives notification, if you don't copy to local variable:
   2. Check it for null, to prevent NullReferenceException

   But now, we can use ?. operator for null-checking. And if I use it, Resharper is idle:

   So, question is: should I copy event ProperyChanged to local variable, if I use null-conditional operator?

   解决方案

   should I copy event ProperyChanged to local variable, if I use null-conditional operator?

   No, there's no need. In fact, one of the main reasons the null-conditional operator was introduced was to simplify code using this pattern. It has the same effect as copying the source value to a local variable and inherently avoids the "check and use" concurrency trap that the "copy to local variable" technique is intended to address.

   See related posts:
   Invoking Events, h(args) vs EventName?.Invoke() (almost an exact duplicate…it does approach the question from a slightly different angle though)
   Why should I check for null before I invoke the custom event?
   Raising C# events with an extension method - is it bad?
   Is there any reason to assign an event to a local variable before raising it?

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