Should I return IEnumerable or IQueryable from my DAL?

Question

I know this could be opinion, but I\'m looking for best practices.

As I understand, IQueryable implements IEnumerable, so

Answer 1

It depends on what behavior you want.

• Returning an IList tells the caller that they've received all of the data they've requested
• Returning an IEnumerable tells the caller that they'll need to iterate over the result and it might be lazily loaded.
• Returning an IQueryable tells the caller that the result is backed by a Linq provider that can handle certain classes of queries, putting the burden on the caller to form a performant query.

While the latter gives the caller a lot of flexibility (assuming your repository fully supports it), it's the hardest to test and, arguably, the least deterministic.