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What is the difference between IEnumerable, ICollection and IList?

C# Collections Intermediate 1–2 Yrs2–3 Yrs Conceptual 1 min read

Quick Answer

They form a hierarchy of capability. IEnumerable<T> — iterate only (foreach). ICollection<T> — adds Count, Add, Remove, Contains. IList<T> — adds index access and Insert/RemoveAt. Expose the least capable interface a caller needs.

It's a capability ladder. IEnumerable<T> just lets you iterate. ICollection<T> adds count, add, remove and contains. IList<T> adds indexing and insert-at-position. The principle I follow is to expose the least powerful one that does the job — return IEnumerable<T> or IReadOnlyList<T> from a method so callers can't mutate my internal collection, and only take IList<T> as a parameter when the method really does add or remove items.

  • Return IEnumerable<T> from a method when the caller should only read/iterate — it also allows lazy/streamed results.
  • Return IReadOnlyList<T> / IReadOnlyCollection<T> when the caller needs Count or indexing but shouldn't mutate.
  • Use ICollection<T> / IList<T> for parameters where the method genuinely adds or removes items.
  • Beware: returning IEnumerable<T> that's actually a deferred query — the caller may enumerate it multiple times and re-run it. Materialise if needed.
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