CODEWITHSUNDEEP
c#events
Mr.Pe
Mr.Pe

Reputation: 739

Enum equality (and EventHandlerList's keys)

Subtitle: Can EventHandlerList key's Type be something else than object?

I wanted to use an enum store the keys I would like to have in an EventHandler.

public enum EventKey
{
    OnBark, OnCry
}

public EventHandlerList EventList = new EventHandlerList();

public event ComplaintEventHandler OnBark
{
    add
    {
        EventList.AddHandler(EventKey.OnBark, value);
    }
    remove
    {
        EventList.RemoveHandler(EventKey.OnBark, value);
    }
}

var handler = EventList[eventKey] as ComplaintEventHandler;

>

handler = null

As it turns out it does not work. But it works if I use keys declared like (as shown on):

static object EventKeyOnTap = new object();

After reading some mscorlib's code I see that the problem comes from next.key == key in 

private EventHandlerList.ListEntry Find(object key)
{
    EventHandlerList.ListEntry next = this.head;

    while (next != null && next.key != key)
    {
        next = next.next;
    }
    return next;
}

Both compared keys are from my Enum, but they are not equal! I guess it comes from some implicit casts to object happening (the key stored in the list is of type object) but am not fluent enough with such low-level concepts.

Is my guess right?

What is the best way to use an Enum as a key in an EventHandlerList?

For now I will create my own EventHandlerList with Enum as key Type.

For now I created my own EventHandlerList with a constructor taking a Func<object, object, bool> which I then use in place of the equality comparaison aforementioned.

Upvotes: 2

Views: 640

Answers (1)

Alex Zhevzhik
Alex Zhevzhik

Reputation: 3407

Try this code. Can you explain output? 

var bark1 = (object)EventKey.OnBark;
var bark2 = (object)EventKey.OnBark;

Console.WriteLine(bark1 != bark2);
Console.WriteLine(bark1.Equals(bark2));

If yes, I don't know why you asked this question. If no, you definitely should get aware of value types, reference types and boxing.

In short, AddHandler method accepts object parameter, hence your key (which is value type) is boxed when you call: 

EventList.AddHandler(EventKey.OnBark, value);

If you call this method twice with the same enum key, key will be boxed twice, and two different objects in heap will be actually created.

That's why this check next.key != key inside Find method fails (it compares addresses of two separate objects in heap).

EventHandlerList is sealed class, so you cannot affect its guts, but in your own code you could handle this situation with better check: 

next.key.Equals(key)

Upvotes: 3

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