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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? 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? 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?
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C# › Collections

What is the difference between IEnumerable<T>, ICollection<T>, and IList<T> in the collection interface hierarchy?

Intermediate

Answer

IEnumerable<T> only guarantees the ability to ITERATE (foreach); ICollection<T> extends it with Count, Add, Remove, and Contains; IList<T> extends ICollection<T> further with INDEX-based access (this[int index]) and Insert/RemoveAt — each level adds more capability at the cost of requiring more from the underlying implementation.
void ProcessReadOnly(IEnumerable<int> items) { /* can only iterate */ }
void ProcessCountable(ICollection<int> items) { Console.WriteLine(items.Count); }
void ProcessIndexable(IList<int> items) { Console.WriteLine(items[0]); }
Real-world example Accepting the loosest interface (IEnumerable<T>) a method's parameter genuinely needs, maximizing which concrete collection types can be passed in.

Common follow-ups: Why is it generally considered good practice to accept the LEAST specific interface your method actually requires?

Interfaces & Abstract Classes
All 16 Collections questions