Advanced Collections in C#

Advanced Collections in C#: HashSet, Queues, Stacks.

Updated November 3, 2026

1. HashSet (Journeyman): A HashSet<T> holds each value once, answers "is it in here?" fast, and has no index or guaranteed order. Add tells you whether a value was new.
2. Queues (Journeyman): A Queue<T> hands values back in the order they arrived, first in, first out. Enqueue adds to the back of the line, Dequeue takes from the front, and Peek looks without taking.
3. Stacks (Journeyman): A Stack<T> hands values back newest first, last in, first out. Push adds to the top, Pop takes from the top, and Peek looks without taking. Undo is the classic use.