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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 List<T> and an array (T[]) in terms of resizing? 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 Dictionary<TKey, TValue> and List<T> in terms of lookup performance?

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Answer

Dictionary<TKey, TValue> provides average O(1) constant-time lookups by key via hashing; List<T> requires an O(n) linear scan to find an element unless you already know its exact index — Dictionary is the right choice whenever you need fast lookups by a unique identifier.
Dictionary<string, int> ages = new() { ["Sam"] = 30, ["Alex"] = 25 };
int samAge = ages["Sam"]; // O(1) lookup

List<(string Name, int Age)> list = new() { ("Sam", 30), ("Alex", 25) };
int samAge2 = list.First(p => p.Name == "Sam").Age; // O(n) linear scan
Real-world example Storing a cache of user sessions keyed by session ID, where fast lookup by ID is essential.

Common follow-ups: What happens if you try to access a Dictionary key that doesn't exist, using the indexer versus TryGetValue?

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All 16 Collections questions