LINQ .FromCache().ToList()?

Question

When using .FromCache() on an IQueryable result set, should I additionally call .ToList(), or can I just return the IEnumerable<> returned by the materialized query with FromCache?

Answer 1

I am assuming you are using a derivative of the code from http://petemontgomery.wordpress.com/2008/08/07/caching-the-results-of-linq-queries/ . If you look at the FromCache implementation, you will see the that the query.ToList() is already called. This means that the evaluated list is what is cached. So,

You do NOT need to call ToList()

Answer 2

That depends entirely on what you want to do with it. If you're just going to foreach over it once then you may as well just leave it as an IEnumerable. There's no need to build up a list just to discard it right away.

If you plan to iterate over it multiple times it's probably best to ToList it, so that you're not accessing the underlying IQueryable multiple times. You should also ToList it if it's possible for the underlying query to change over time and you don't want those changes to be reflected in your query.

If you are likely to not need to iterate all of the items (you may end up stopping after the first item, or half way, or something like that) then it's probably best to leave it as an IEnumerable to potentially avoid even fetching some amount of data in the first place.

If the method has no idea how it's going to be used, and it's just a helper method that will be used by not-yet-written code, then consider returning IEnumerable. The caller can call ToList on it if they have a compelling reason to turn it into a list.

For me, as a general rule, I leave such queries as IEnumerable unless I have some compelling reason to make it a List.