c# - C# 中的错误代码通过 CLR 书

Question

我正在读这本书并被困在这里:

public static class EventArgExtensions { 
   public static void Raise<TEventArgs>(this TEventArgs e,  
      Object sender, ref EventHandler<TEventArgs> eventDelegate)  
      where TEventArgs : EventArgs { 

      // Copy a reference to the delegate field now into a temporary field for thread safety  
      EventHandler<TEventArgs> temp =  
         Interlocked.CompareExchange(ref eventDelegate, null, null); 

      // If any methods registered interest with our event, notify them   
      if (temp ! = null) temp(sender, e); 
   } 
}

特别是 MSDN 文档说

public static object CompareExchange(ref object location1, object value, object comparand)

Member of System.Threading.Interlocked

Summary:
Compares two objects for reference equality and, if they are equal, replaces one of the objects.

Parameters:
location1: The destination object that is compared with comparand and possibly replaced.
value: The object that replaces the destination object if the comparison results in equality.
comparand: The object that is compared to the object at location1.

Returns:
The original value in location1.

Exceptions:
System.ArgumentNullException: The address of location1 is a null pointer.

这意味着这是错误的:

 EventHandler<TEventArgs> temp =  
             Interlocked.CompareExchange(ref eventDelegate, null, null); 

          // If any methods registered interest with our event, notify them   
          if (temp ! = null) temp(sender, e); 

因为我需要检查 是什么通过进入 CompareExchange 而不是输出。

我错过了什么吗？

Answer 1

不，代码很好。它实际上只是一种执行 volatile 读取的方式。只有将 null 替换为 null 时才会返回 null - 即如果 eventDelegate一片空白。