C#中的错误代码通过CLR Book

Question

我正在读这本书,我被困在这里:

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 doc说的那样

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

System.Threading.Interlocked的成员

摘要:
比较两个对象以获得引用相等性,如果它们相等,则替换其中一个对象.

参数:
location1:与comparand进行比较并可能被替换的目标对象.
value:如果比较结果相等,则替换目标对象的对象.
comparand:与location1处的对象进行比较的对象.

返回:
location1中的原始值.

异常:
System.ArgumentNullException:location1的地址是空指针.

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); 

因为我需要检查传递给CompareExchange的内容而不是输出.

我错过了什么吗？

Answer 1

不,代码很好.它实际上只是执行易失性读取的一种方式.如果用null替换null,它将只返回null - 即if eventDelegate为null.