public class EntityExtensionDictionary<TKey,TValue> : Dictionary<TKey,TValue>
    {
        private Func<TKey, TValue> _ObjectCreator;

        public EntityExtensionDictionary(Func<TKey, TValue> objectCreator)
        {
            _ObjectCreator = objectCreator;
        }
        public EntityExtensionDictionary(IDictionary<TKey, TValue> data, Func<TKey, TValue> objectCreator)
            : this(objectCreator)
        {
            foreach (var kvp in data)
                this.Add(kvp.Key, kvp.Value);
        }

        public new TValue this[TKey key]
        {
            get
            {
                if (base.Keys.Contains(key))
                    return base[key];
                var newValue = _ObjectCreator(key);
                base.Add(key, newValue);
                return newValue;
            }
            set { base[key] = value; }
        }
    }