Chapter 15 covered the standard query operators—that is, the extension methods on
IEnumerable<T> that provide methods common to all collections. However, these operators do not make all collections equally suited for all tasks; there is still a need for different collection types. Some collections are better suited to searching by key, whereas others are better suited to accessing items by position. Some collections act like queues: The first element in is the first out. Others are more like stacks: The first element in is the last out. Others are not ordered at all.