How does the HashSet<T> differ from a List<T> or Dictionary<TKey, TValue>?

Short answer: Feature HashSet<T> List<T> Dictionary<TKey, TValue> Allows duplicates? No Yes Keys: No; Values: Yes Order No guaranteed order Maintains insertion order No guaranteed order Lookup speed O(1) average O(n) O(1) average for keys Key-value pairs No (only values) No Yes (key-value pairs)

Real-world example (ShopNest)

In ShopNest, an order page loads products with List<Product> so you can Add items to the cart and access them by index. Use IEnumerable<T> when you only need to loop (for example, printing invoice lines).

Say this in the interview

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.