Unlike a List<> …
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A HashSet is a List with no duplicate members.
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Because a HashSet is constrained to contain only unique entries, the internal structure is optimised for searching (compared with a list) – it is considerably faster
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Adding to a HashSet returns a boolean – false if addition fails due to already existing in Set
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Can perform mathematical set operations against a Set: Union/Intersection/IsSubsetOf etc.
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HashSet doesn’t implement IList only ICollection
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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!