What is a HashSet<T> in C# and when would you use it?

Short answer: HashSet<T> is a generic collection that stores unique elements with no particular order. It is optimized for fast lookups, additions, and deletions. Use case: When you want to store a collection of unique items without duplicates, such as user IDs, tags, or keywords. HashSet<int> uniqueNumbers = new HashSet<int> { 1, 2, 3 }; uniqueNumbers.Add(4);

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1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.