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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? 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? How would you choose between List<T>, LinkedList<T>, and ImmutableList<T> based on their different Big-O characteristics for common operations?
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C# › Collections

How does Queue<T> differ from Stack<T> in terms of the order items are removed?

Intermediate

Answer

Queue<T> is First-In-First-Out (FIFO) — items are removed (Dequeue) in the same order they were added (Enqueue); Stack<T> is Last-In-First-Out (LIFO) — the most recently added item (Push) is the first one removed (Pop).
Queue<string> queue = new();
queue.Enqueue("first"); queue.Enqueue("second");
Console.WriteLine(queue.Dequeue()); // 'first'

Stack<string> stack = new();
stack.Push("first"); stack.Push("second");
Console.WriteLine(stack.Pop()); // 'second'
Real-world example Using Queue<T> for a print job or task processing queue; Stack<T> for an undo/redo history or a depth-first traversal.

Common follow-ups: Which of these two collections would you use to implement a breadth-first graph traversal?

Iterators & yield return
All 16 Collections questions