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> :)

Answer 2

DrJokepu said that it might be difficult to implement a wrapper for Keys Collection. But, in this particular case, I think the implementation is not so difficult because, as we know, this is a read-only wrapper.

This allows us to ignore some methods that, in other case, would be hard to implement.

Here's a quick implementation of the wrapper for Dictionary.KeyCollection :

class MyListWrapper<T, TValue> : IList<T>
{
    private Dictionary<T, TValue>.KeyCollection keys;

    public MyListWrapper(Dictionary<T, TValue>.KeyCollection keys)
    {
        this.keys = keys;
    }

    #region IList<T> Members

    public int IndexOf(T item)
    {
        if (item == null)
            throw new ArgumentNullException();
        IEnumerator<T> e = keys.GetEnumerator();
        int i = 0;
        while (e.MoveNext())
        {
            if (e.Current.Equals(item))
                return i;
            i++;
        }
        throw new Exception("Item not found!");
    }

    public void Insert(int index, T item)
    {
        throw new NotImplementedException();
    }

    public void RemoveAt(int index)
    {
        throw new NotImplementedException();
    }

    public T this[int index]
    {
        get
        {
            IEnumerator<T> e = keys.GetEnumerator();
            if (index < 0 || index > keys.Count)
                throw new IndexOutOfRangeException();
            int i = 0;
            while (e.MoveNext() && i != index)
            {
                i++;
            }
            return e.Current;
        }
        set
        {
            throw new NotImplementedException();
        }
    }

    #endregion

    #region ICollection<T> Members

    public void Add(T item)
    {
        throw new NotImplementedException();
    }

    public void Clear()
    {
        throw new NotImplementedException();
    }

    public bool Contains(T item)
    {
        return keys.Contains(item);
    }

    public void CopyTo(T[] array, int arrayIndex)
    {
        keys.CopyTo(array, arrayIndex);
    }

    public int Count
    {
        get { return keys.Count; }
    }

    public bool IsReadOnly
    {
        get { return true; }
    }

    public bool Remove(T item)
    {
        throw new NotImplementedException();
    }

    #endregion

    #region IEnumerable<T> Members

    public IEnumerator<T> GetEnumerator()
    {
        return keys.GetEnumerator();
    }

    #endregion

    #region IEnumerable Members

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return keys.GetEnumerator();
    }

    #endregion
}

This might not be the best implementation for these methods :) but it was just for proving that this might be done.

Answer 3

Assuming you are using C# 3.0 and you have:

Dictionary< T,S > d;

Then

ReadOnlyCollection< T > r = new ReadOnlyCollection< T >( d.Keys.ToList() );

You will also need to import the System.Linq namespace.

Answer 4

Unfortunately you cannot to that direcly as far as I know as KeyCollection<T> does not expose anything that would allow you to do this easily.

You could, however, subclass ReadOnlyCollection<T> so that its constructor receives the dictionary itself and override the appropriate methods so that it exposes the Dictionary's items as if they were its own items.

Answer 5

For the record, in .NET 4.6, the KeyCollection<T> implements IReadOnlyCollection<T>, so if you use that interface, you can still reflect changes to the dictionary, still get O(1) contains, and because the interface is covariant, you can return IReadOnlyCollection<some base type>

Answer 6

It's ugly, but this will do it

Dictionary<int,string> dict = new Dictionary<int, string>();
...
ReadOnlyCollection<int> roc = new ReadOnlyCollection<int>((new List<int>((IEnumerable<int>)dict.Keys)));