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Asked: 2026-05-31T13:31:38+00:00

I need to have an ability to have unique items in a collection. I

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I need to have an ability to have unique items in a collection.

I was going to use a Dictionary so I could use the ContainsKey method but I thought it would be a waste as I wouldnt use the Value property of the Key/Value pair.

I came across the HashSet<T> which looks very promising. The only thing I can find that I can’t find in the List<T> docs is that HashSet<T> is unordered. I think that is fine, I assume it means its not ordered using a IEqualityComparer. As long as the order in which items are added are in the same index position I think it will be ok as I have to do duplicate checking hence the hashset and then check all entries are sequential.

Is there anything else I have missed comparing the two types?

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  1. Editorial Team

    No, importantly HashSet<T> doesn’t have any concept of ordering or indexing – a list conceptually has slots 0….n-1, whereas a set is “just a set”.

    I think that is fine, I assume it means its not ordered using a IEqualityComparer.

    IEqualityComparer isn’t used for ordering anyway – it only talks about equality and hash codes. HashSet<T> isn’t ordered by either an element comparison (as, say, SortedSet<T> is) or insertion order.

    As long as the order in which items are added are in the same index position I think it will be ok.

    There is no index position, and when you iterate over a HashSet<T> there’s no guarantee you’ll get them back in the order in which you added them. If you’re even thinking about ordering, HashSet<T> isn’t what you’re after.

    Then again, all of this is also true of Dictionary<TKey, TValue> – you shouldn’t make any assumptions about ordering there, either.

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