How to get a ReadOnlyCollection<T> of the Keys in a Dictionary<T, S>

Question

My class contains a Dictionary<T, S> dict, and I want to expose a ReadOnlyCollection<T> of the keys. How can I do this without copying the Dictionary<T, S>.KeyCollection dict.Keys to an array and then exposing the array as a ReadOnlyCollection?

I want the ReadOnlyCollection to be a proper wrapper, ie. to reflect changes in the underlying Dictionary, and as I understand it copying the collection to an array will not do this (as well as seeming inefficient - I don't actually want a new collection, just to expose the underlying collection of keys...). Any ideas would be much appreciated!

Edit: I'm using C# 2.0, so don't have extension methods such as .ToList (easily) available.

Answer 1

If you really want to use ReadOnlyCollection<T>, the issue is that the constructor of ReadOnlyCollection<T> takes an IList<T>, while the KeyCollection of the Dictionary is only a ICollection<T>.

So if you want to wrap the KeyCollection in a ReadOnlyCollection, you'll have to create an adapter (or wrapper) type, implementing IList<T>, wrapping the KeyCollection. So it would look like:

var dictionary = ...;
var readonly_keys = new ReadOnlyCollection<T> (new CollectionListWrapper<T> (dictionary.Keys)
);

Not very elegant though, especially as the KeyCollection is already a readonly collection, and that you could simply pass it around as an ICollection<T> :)