| IndexByTSource, TKey(IEnumerableTSource, FuncTSource, TKey) | Applies a key-generating function to each element of a sequence and returns a sequence that contains the elements of the original sequence as well its key and index inside the group of its key. |
| IndexByTSource, TKey(IEnumerableTSource, FuncTSource, TKey, IEqualityComparerTKey) | Applies a key-generating function to each element of a sequence and returns a sequence that contains the elements of the original sequence as well its key and index inside the group of its key. An additional parameter specifies a comparer to use for testing the equivalence of keys. |