Why is IList not deferred execution?

Question

As I understand it IEnumerable and IQueryable are deferred execution. Why wouldn't it be of benefit for IList to also support deferred execution?

Answer 1

The longer I think about it, the more I question whether the whole idea of "deferred execution" is actually of pedagogic value at all.

I'll answer your question by denying it. IEnumerable<T>, IQueryable<T> and IList<T> do not in any way represent "deferred" or "eager" calculations.

Rather, interfaces represent an ability to provide a service. IEnumerable<T> represents the service "I can provide a sequence, possibly infinite, of items of type T, one at a time". IQueryable<T> represents the service "I can represent a query against a data source, and provide the results of that query on demand". IList<T> represents the service "I can provide random access to a possibly mutable, finite-sized list of items of type T".

None of those services say anything about the implementation details of the service providers. The provider of an IList<T> service could be entirely lazy; the provider of an IQueryable<T> service could be entirely eager. If you want to make a deferred-execution IList<T>, you go right ahead. No one is stopping you!