All 16 Collections questions
More in Collections
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 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?
Advertisement
C# › Collections

How do you avoid multiple enumeration of an IEnumerable?

Advanced

Answer

Deferred sequences re-run their query each time you enumerate them; materialise once with ToList()/ToArray() if you iterate more than once or the source is expensive.
var q = Load().Where(x => x.Active);
var list = q.ToList(); // enumerate once
var count = list.Count; foreach (var x in list) {}
Real-world example Enumerating a database-backed IQueryable twice issues two SQL queries — materialise first.
All 16 Collections questions