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

What is the difference between List<T> and an array (T[]) in terms of resizing?

Beginner

Answer

An array has a FIXED size determined at creation and can never grow or shrink; List<T> is a dynamically-resizable collection built on top of an internal array that automatically grows (by reallocating and copying to a larger array) as you add elements beyond its current capacity.
int[] fixedArray = new int[3]; // always exactly 3 elements
// fixedArray[3] = 4; // IndexOutOfRangeException

List<int> list = new List<int>();
list.Add(1); list.Add(2); list.Add(3); list.Add(4); // grows automatically
Real-world example Using List<T> for a shopping cart's items (unknown count ahead of time) versus a fixed array for exactly 7 days of the week.

Common follow-ups: What happens internally when a List<T>'s capacity is exceeded and it needs to grow?

Arrays Span<T> & Memory<T>
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