All 16 Collections questions
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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? What does HashSet<T> provide and when do you use it? 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? What is the performance and thread-safety difference between ConcurrentDictionary<TKey,TValue> and a regular Dictionary<TKey,TValue> guarded by a manual lock?
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C# › Collections

When would you choose a HashSet<T> over a List<T>, and what guarantee does it provide?

Intermediate

Answer

HashSet<T> guarantees every element is UNIQUE (adding a duplicate is a silent no-op) and provides O(1) average .Contains() lookups, unlike List<T>'s O(n) linear .Contains() scan and lack of uniqueness enforcement — ideal when you need fast membership testing or automatic deduplication.
HashSet<int> visitedIds = new();
if (visitedIds.Add(userId)) {
  ProcessUser(userId); // only runs the first time this ID is seen
}
Real-world example Tracking which user IDs have already been processed in a batch job, avoiding duplicate work efficiently.

Common follow-ups: Does HashSet<T> preserve insertion order the way a List<T> does?

Generics
All 16 Collections questions