当事件订阅者取消订阅 class 的事件时是否可以收到通知？

Question

是否有可能在执行 Place1 行时收到通知并执行 Place2？

interface ICat
{
    void StartWork();
    public event EventHandler WorkDone;
}
class Cat : ICat
{
    public event EventHandler WorkDone;

    public void StartWork()
    {
    }

    void OnUnsubscribed()
    {
        //Place2
    }
}

class CatUser
{
    CatUser()
    {
        var cat = new Cat();
        cat.WorkDone += Cat_WorkDone;
        cat.WorkDone -= Cat_WorkDone; //<--- Place1
    }

    private void Cat_WorkDone(object sender, EventArgs e)
    {
    }
}

Answer 1

你应该这样做：

 interface ICat
{
    void StartWork();
    event EventHandler WorkDone;
}
class Cat : ICat
{
    public event EventHandler WorkDone;

    public void StartWork()
    {
    }

    void OnUnsubscribed()
    {
        //Place2
    }

    internal void UnSubscribe(System.EventHandler cat_WorkDone)
    {
        this.WorkDone -= cat_WorkDone;
        OnUnsubscribed();
    }
}

class CatUser
{
    CatUser()
    {
        var cat = new Cat();
        cat.WorkDone += Cat_WorkDone;
        cat.UnSubscribe(Cat_WorkDone);
    }

    private void Cat_WorkDone(object sender, EventArgs e)
    {
    }
}

Answer 2

是的，事件之于委托就像属性之于支持字段。您可以将事件写为

private EventHandler _workDone;
public event EventHandler WorkDone
{
    add
    {
        // Event subscriber subscribes
        _workDone += value;
    }
    remove
    {
        // Event subscriber unsubscribes
        _workDone -= value;
    }
}

事件通常是自动实现的，并在后台自动创建添加和删除访问器..

另请参阅：

How to implement custom event accessors (C# Programming Guide)
.NET Matters / Event Accessors (Microsoft Docs)（显示 Delegate.Remove 的示例，请参阅@madreflection 的评论）。
add (C# Reference)
remove (C# Reference)

当事件订阅者取消订阅 class 的事件时是否可以收到通知？

Is it possible to get notified when an event subscriber unscribes from the class's event?

c#

event-handling