When should I use the HashSet type?

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The important thing about HashSet<T> is right there in the name: it’s a set. The only things you can do with a single set is to establish what its members are, and to check whether an item is a member.

Asking if you can retrieve a single element (e.g. set[45]) is misunderstanding the concept of the set. There’s no such thing as the 45th element of a set. Items in a set have no ordering. The sets {1, 2, 3} and {2, 3, 1} are identical in every respect because they have the same membership, and membership is all that matters.

It’s somewhat dangerous to iterate over a HashSet<T> because doing so imposes an order on the items in the set. That order is not really a property of the set. You should not rely on it. If ordering of the items in a collection is important to you, that collection isn’t a set.

Sets are really limited and with unique members. On the other hand, they’re really fast.

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