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What does HashSet<T> provide and when do you use it? 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? 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? How would you choose between List<T>, LinkedList<T>, and ImmutableList<T> based on their different Big-O characteristics for common operations? How does the 'yield return'-based lazy evaluation of collections (like LINQ or a custom iterator) differ from eagerly materializing a full collection, in terms of memory usage? How would you implement a custom, strongly-typed, read-only wrapper around a mutable collection to safely expose it from a class's public API?
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C# › Collections

When would you reach for a ConcurrentDictionary or an immutable collection?

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Use ConcurrentDictionary for thread-safe reads/writes without external locks; use ImmutableList/Dictionary when you want safe sharing across threads via copy-on-write snapshots.
var cache = new ConcurrentDictionary<int,User>();
cache.GetOrAdd(id, Load);
Real-world example A shared in-memory cache updated by many request threads uses ConcurrentDictionary.
All 16 Collections questions