IEnumerable, ICollection, IList and IQueryable: What Each Collection Interface Promises

Wed, 21 Oct 2026 12:40:00 +0000

Open almost any C# codebase and you will find method signatures that take an IEnumerable<T>, entities with ICollection<T> navigation properties, repositories returning IQueryable<T>, and the occasional IList<T> or IReadOnlyList<T>. They are easy to treat as interchangeable names for “a bunch of items”, and the code usually compiles whichever one you pick. Yet each interface makes a different promise, and choosing the wrong one causes real bugs: a query that runs three times, a database table loaded into memory to be filtered in C#, an exception thrown far from the line that caused it, or a “read-only” list that changes under your feet.

Deferred Execution on Artur Rios Lab

IEnumerable, ICollection, IList and IQueryable: What Each Collection Interface Promises