Описание тега hashset

HashSet<T> was introduced in.NET Framework 3.5 as part of the System.Collections.Generic namespace. HashSet is an unordered collection containing unique elements and provides a set of standard set operators such as intersection and union (plus many more). It has the standard collection operations Add (although this method returns a Boolean indicating whether or not that element already existed in the collection), Remove, and Contains, but because it uses a hash-based implementation for object identity, these operations are immediately accessible without looping the entire list as occurs with the List<T> collection for example (O(1) rather than O(n)).

Source: Linq to Objects using C# 4.0, Troy Magennis, Addison Wesley, 2010, Pearson Education Inc, ISBN-13: 978-0-321-63700-0

References

• HashSet Class on Docs
• "Define: What is a HashSet?" on Stackru