| CountByTSource, TKey(IEnumerableTSource, FuncTSource, TKey) | Applies a key-generating function to each element of a sequence and returns a sequence of unique keys and their number of occurrences in the original sequence. |
| CountByTSource, TKey(IEnumerableTSource, FuncTSource, TKey, IEqualityComparerTKey) | Applies a key-generating function to each element of a sequence and returns a sequence of unique keys and their number of occurrences in the original sequence. An additional argument specifies a comparer to use for testing equivalence of keys. |