Linq Dreams - GroupAndAggregate

 

 public static
  IEnumerable<KeyValuePair<TKey, TOutput>>
  GroupAndAggregate<TElement, TKey, TOutput>   
   (this IEnumerable<TElement> source, Func<TElement, TKey> keyMapping,
   Func<TElement, TOutput> firstElementMapping,
   Func<TElement, TOutput, TOutput> subsequentElementMapping)
 {
  Dictionary<TKey, TOutput> dictionary = new Dictionary<TKey, TOutput>();
  foreach (TElement element in source)
  {
   TKey key = keyMapping(element);
   TOutput output = default(TOutput);
   if (dictionary.TryGetValue(key, out output))
   {
    output = subsequentElementMapping(element, output);
   }
   else
   {
    output = firstElementMapping(element);
   }
   dictionary[key] = output;
  }
  return dictionary;
 }