public static TResult GetValue<TKey, TValue, TResult>(this IDictionary<TKey, TValue> source, 
                                                          TKey key, 
                                                          Func<TValue, TResult> callback,
                                                          Func<TKey, TResult> callbackIfEmpty)
    {
      if (!source.ContainsKey(key))
      {
        return callbackIfEmpty(key);
      }
      var result = source[key];

      return callback(result);
    }