Lazy dictionary

gistfile1.cs

public class LazyDictionary<TKey, TValue>
{
    private readonly Func<TKey, TValue> _factory;
    private readonly Dictionary<TKey, TValue> _cache;

    public LazyDictionary(Func<TKey, TValue> factory)
    {
        _factory = factory;
        _cache = new Dictionary<TKey, TValue>();
    }

    public TValue this[TKey key]
    {
        get
        {
            if (!_cache.ContainsKey(key))
            {
                _cache.Add(key, _factory(key));
            }

            return _cache[key];
        }
    }
}