insert keys automatically dictionary in csharp

                
                        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(); }
}