internal class AutoKeyDictionary<TKey, TValue> : IEnumerable<KeyValuePair<TKey, TValue>>, IEnumerable

{

    private readonly Dictionary<TKey, TValue> inner;

    private readonly Func<TKey, TKey> incrementor;

    private readonly Stack<TKey> freeKeys;

    private readonly TKey keySeed;

    private TKey currentKey;

​

    public AutoKeyDictionary(TKey keySeed, Func<TKey, TKey> incrementor) 

    {

        if (keySeed == null)

            throw new ArgumentNullException("keySeed");

​

        if (incrementor == null)

            throw new ArgumentNullException("incrementor");

​

        inner = new Dictionary<TKey, TValue>();

        freeKeys = new Stack<TKey>();

        currentKey = keySeed;

    }

​

    public TKey Add(TValue value) //returns the used key

    {

        TKey usedKey;

​

        if (freeKeys.Count > 0)

        {

            usedKey = freeKeys.Pop();

            inner.Add(usedKey, value);

        }

        else

        {

            usedKey = currentKey;

            inner.Add(usedKey, value);

            currentKey = incrementor(currentKey);

        }

​

        return usedKey;

    }

​

    public void Clear()

    {

        inner.Clear();

        freeKeys.Clear();

        currentKey = keySeed;

    }

​

    public bool Remove(TKey key)

    {

        if (inner.Remove(key))

        {

            if (inner.Count > 0)

            {

                freeKeys.Push(key);

            }

            else

            {

                freeKeys.Clear();

                currentKey = keySeed;

            }

​

            return true;

        }

​

        return false;

    }

​

    public bool TryGetValue(TKey key, out TValue value) { return inner.TryGetValue(key, out value); }

    public TValue this[TKey key] { get {return inner[key];} set{inner[key] = value;} }

    public bool ContainsKey(TKey key) { return inner.ContainsKey(key); }

    public bool ContainsValue(TValue value) { return inner.ContainsValue (value); }

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

    public Dictionary<TKey,TValue>.KeyCollection Keys { get { return inner.Keys; } }

    public Dictionary<TKey, TValue>.ValueCollection Values { get { return inner.Values; } }

    public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator() { return inner.GetEnumerator(); }

    IEnumerator IEnumerable.GetEnumerator() { return ((IEnumerable)inner).GetEnumerator(); }

}