FirstOrDefault() Observable implementation on changing list

[szekelymatyas]

Just want to get the first or default item of a changing collaction. Here is my first thought:

public static IObservable<T?> FirstOrDefault<T>(this IObservableList<T> list)
{
    return list.CountChanged.Select(x => list.Items.FirstOrDefault()).Distinct();
}

Is it okey? (probably not, but i ask it anyway)
Is it thread safe? (same⬆️)
Is this fast enough? (same⬆️)
How can i do it with Connect and IObservable<IChangeSet<>>? (propably gonna miss some edge cases if i do it)
Is there an existing implementation?