Difference between IQueryable and Queryable

Question

I haven't quite got my head around interfaces so thought I'd word the question in a way that'd help me better understand it.

I'm following a tutorial which has had me make an IQueryable. Why couldn't I just make a Queryable?

Answer 1

Queryable is just a static class that contains extension methods to the IQueryable<T> interface. You wouldn't use Queryable directly in your code but rather invoke its methods given an IQueryable<T> instance.

Answer 2

Queryable is a static class that provides some convenient and useful methods to anything implementing IQueryable. You can't make it because it's already made. You need to make a new class that actually does what you want it to do, and implement IQueryable so other code written to use IQueryable (including Queryable) knows how to use it.