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When would you use a List<T> vs an array? 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? 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?
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When should you use a Dictionary<TKey,TValue>?

Beginner

Answer

When you need fast key-based lookups (O(1) average). Keys are unique; use TryGetValue to avoid exceptions on missing keys.
var prices = new Dictionary<string,decimal>{["pen"]=1.5m};
if (prices.TryGetValue("pen", out var p)) { }
Real-world example Caching users by Id or counting word frequencies.
All 16 Collections questions