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Unlike a List<>

1. A HashSet is a List with no duplicate members.
2. Because a HashSet is constrained to contain only unique entries, the internal structure is optimized for searching (compared with a list) – it is considerably faster
3. Adding to a HashSet returns a boolean – false if addition fails due to already existing in Set
4. Can perform mathematical set operations against a Set: Union/Intersection/IsSubsetOf etc.
5. HashSet doesn’t implement IList only ICollection
6. You cannot use indices with a HashSet, only enumerators.

The main reason to use a HashSet would be if you are interested in performing Set operations.

Given 2 sets: hashSet1 and hashSet2

//returns a list of distinct items in both sets
HashSet set3 = set1.Union(set2);

flies in comparison with an equivalent operation using LINQ. It’s also neater to write!

Thanks to BonyT for this answer.