# HashSet

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.

![Cheat sheet: HashSet](https://fsdtrainingmedia.blob.core.windows.net/public/images/2026/11/2f8406c79a1a4d529a4a6eff30d8b0e5.webp)

- Category: C#
- Level: Journeyman
- Tags: c-sharp, HashSet<T>
- Updated: November 3, 2026

Source: https://www.ask-codey.com/cheat-sheets/advanced-collections-in-csharp-hashset-cheat-sheet