C＃模式prevent事件处理程序两次挂钩 [英] C# pattern to prevent an event handler hooked twice

问题描述

的复制：<一href=\"http://stackoverflow.com/questions/367523/how-to-ensure-an-event-is-only-subscribed-to-once\">How确保一次的事件只认购
和已添加的事件处理程序？

Duplicate of: How to ensure an event is only subscribed to once and Has an event handler already been added?

我有一个单身的提供了一些服务，我的课挂接到它的一些事件，有时一类是挂钩两次的事件，然后被调用两次。
我正在寻找的发生古典的方式来prevent这一点。不知何故，我需要检查，如果我已经迷上这个事件......

I have a singleton that provides some service and my classes hook into some events on it, sometimes a class is hooking twice to the event and then gets called twice. I'm looking for a classical way to prevent this from happening. somehow I need to check if I've already hooked to this event...

推荐答案

明确落实该事件，并检查调用列表。您还需要检查null：

Explicitly implement the event and check the invocation list. You'll also need to check for null:

using System.Linq; // Required for the .Contains call below:

...

private EventHandler foo;
public event EventHandler Foo
{
    add
    {
        if (foo == null || !foo.GetInvocationList().Contains(value))
        {
            foo += value;
        }
    }
    remove
    {
        foo -= value;
    }
}

使用上面的code，如果呼叫者订阅事件多次，它将简单地被忽略。

Using the code above, if a caller subscribes to the event multiple times, it will simply be ignored.

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