Grouping and Joining
Slightly more complex query functionality exists in the family of grouping and joining operators. The number of overloads for those standard query operators may be frightening at first, so let’s take a closer look.
To put elements in buckets with similar properties, grouping operations are used. No fewer than eight overloads exist for the
GroupBy operator, which by themselves can be grouped into some categories. One set of overloads exposes a sequence containing
IGrouping<TKey, TElement> objects, which contain the grouping key and the elements that go with it (because each grouping object is also an
public interface IGrouping<out TKey, out TElement> : IEnumerable<TElement> ...