All 16 Collections questions
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When should you use a Dictionary<TKey,TValue>? What is the difference between List<T> and an array (T[]) in terms of resizing? What is the difference between Dictionary<TKey, TValue> and List<T> in terms of lookup performance? What is the difference between IEnumerable, ICollection and IList? When would you choose a HashSet<T> over a List<T>, and what guarantee does it provide? How does Queue<T> differ from Stack<T> in terms of the order items are removed? What is the difference between IEnumerable<T>, ICollection<T>, and IList<T> in the collection interface hierarchy? When would you reach for a ConcurrentDictionary or an immutable collection? How do you avoid multiple enumeration of an IEnumerable? How does Dictionary<TKey,TValue> handle hash collisions internally, and why does a poor GetHashCode() implementation degrade its performance?
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C# › Collections

What does HashSet<T> provide and when do you use it?

Intermediate

Answer

A set of unique elements with fast Contains/Add and set operations. Adding a duplicate is a no-op.
var seen = new HashSet<int>();
if (!seen.Add(id)) Console.WriteLine("duplicate");
Real-world example De-duplicating ids or checking membership in a loop without O(n) scans.
All 16 Collections questions