DefaultDictionary for C#

DefaultDictionary.cs

class DefaultDictionary<Key, Value> : Dictionary<Key, Value> {
    public new Value this[Key key]
    {
        get
        {
            if (!ContainsKey(key))
            {
                base[key] = default(Value);
            }
            return base[key];
        }
        set
        {
            base[key] = value;
        }
    }
}