-event-只能出现在+ =或 - =的左侧 [英] -event- can only appear on the left hand side of += or -=

问题描述

我有一个循环中的事件。我试图阻止相同的方法被添加到一个事件不止一次。我已经实现了 add 和删除访问器。

但是，我收到一条错误，指出：

ItemsProcessed只能出现在+ =或 - =

当我尝试打电话给他们，即使在同一个班级。

 code> ItemsProcessed（这是新的EventArgs（））; //生成错误
 
公共事件EventHandler ItemsProcessed 
 {
添加
 {
 ItemsProcessed  -  = value; 
 ItemsProcessed + = value; 
} 
删除
 {
 ItemsProcessed  -  = value; 
} 
} 
  

解决方案

一个明确的事件，你需要提供你自己的后台存储 - 一个委托字段或类似于 EventHandlerList 的东西。当前代码是递归的。尝试：

 私人EventHandler itemsProcessed; 
 public event EventHandler ItemsProcessed 
 {
 add 
 {
 itemsProcessed- = value; 
 itemsProcessed + = value; 
} 
 
删除
 {
 itemsProcessed- = value; 
} 
} 
  

然后（并注意到我是一个>小谨慎的即将转向 null 边缘案例重新线程）：

  var snapshot = itemsProcessed; 
 if（snapshot！= null）snapshot（this，EventArgs.Empty）; 
  

随着最新的C＃版本，可以简化：

  itemsProcessed？.Invoke（this，EventArgs.Empty）; 
  

I have an event in a loop. I am trying to prevent the same method being added to an event more than once. I've implemented the add and remove accessors.

However, I get an error stating that:

ItemsProcessed can only appear on the left hand side of += or -=

When I try to call them, even within the same class.

ItemsProcessed(this, new EventArgs()); // Produces error

public event EventHandler ItemsProcessed
{
    add
    {
        ItemsProcessed -= value;
        ItemsProcessed += value;
    }
    remove
    {
        ItemsProcessed -= value;
    }
}

解决方案

With an explicit event, you need to provide your own backing store - either a delegate field or something like EventHandlerList. The current code is recursive. Try:

private EventHandler itemsProcessed;
public event EventHandler ItemsProcessed
{
    add
    {
        itemsProcessed-= value;
        itemsProcessed+= value;
    }

    remove
    {
        itemsProcessed-= value;
    }
}

Then (and noting I'm being a little cautious about the "about to turn null" edge-case re threading):

var snapshot = itemsProcessed;
if(snapshot != null) snapshot(this, EventArgs.Empty);

With more recent C# versions, this can be simplified:

itemsProcessed?.Invoke(this, EventArgs.Empty);

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