C# pattern to prevent an event handler hooked twice

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How to ensure an event is only subscribed to once 6 answers

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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;
    }
}

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

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如果只是首先用-=删除事件，如果没有发现异常则不会抛出

/// -= Removes the event if it has been already added, this prevents multiple firing of the event
((System.Windows.Forms.WebBrowser)sender).Document.Click -= new System.Windows.Forms.HtmlElementEventHandler(testii);
((System.Windows.Forms.WebBrowser)sender).Document.Click += new System.Windows.Forms.HtmlElementEventHandler(testii);

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I've tested each solution and the best one (considering performance) is:

private EventHandler _foo;
public event EventHandler Foo {

    add {
        _foo -= value;
        _foo += value;
    }
    remove {
        _foo -= value;
    }
}

No Linq using required. No need to check for null before cancelling a subscription (see MS EventHandler for details). No need to remember to do the unsubscription everywhere.

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