c# - 注册 C# 事件是线程安全的吗？

Question

具体来说，“+=”操作是原子的吗？ 如果我使用“事件”关键字或只是一个普通的旧代表，这会有什么不同吗？

对于大多数类型，它是读取，然后是“+”运算符，然后是写入。所以，它不是原子的。我想知道代表/事件是否有特殊情况。

这种代码是必要的还是多余的:

Action handler;
object lockObj;
public event Action Handler {
    add { lock(lockObj) { handler += value; } }
    remove { lock(lockObj) { handler -= value; } }
}

Answer 1

是的，自动实现的事件 上的 += 和 -= 运算符是原子的(如果一个库使用自定义事件处理程序很容易不是原子的)。来自 MSDN 杂志文章 .NET Matters: Event Accessors

When the C# compiler generates code for MyClass, the output Microsoft® Intermediate Language (MSIL) is identical in behavior to what would have been produced using code like that in Figure 1.

Figure 1 Expanded Event Implementation

class MyClass
{
    private EventHandler _myEvent;

    public event EventHandler MyEvent
    {
        [MethodImpl(MethodImplOptions.Synchronized)]
        add 
        { 
            _myEvent = (EventHandler)Delegate.Combine(_myEvent, value);
        }
        [MethodImpl(MethodImplOptions.Synchronized)]
        remove 
        { 
            _myEvent = (EventHandler)Delegate.Remove(_myEvent, value); 
        }
    }
    ...
}

[...]

Another use for explicit event implementation is to provide a custom synchronization mechanism (or to remove one). You'll notice in Figure 1 that both the add and remove accessors are adorned with a MethodImplAttribute that specifies that the accessors should be synchronized. For instance events, this attribute is equivalent to wrapping the contents of each accessor with a lock on the current instance:

add { lock(this) _myEvent += value; } 
remove { lock(this) _myEvent -= value; }