The HashSet<T> Class

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)).

Although the operators on HashSet would appear to overlap with the LINQ set operators, Intersect and Union, these HashSet implementations modify the set they were called on, rather than return a new IEnumerable<T> collection, as is the behavior of the LINQ operators. For this reason, the names on the HashSet operators are differentiated as IntersectWith and UnionWith, and the LINQ operators are also available with a HashSet collection as Intersect and Union. This naming distinction avoids naming clashes and also allows the desired behavior to be chosen in a specific case.