select n largest using LINQ

Question

This is likely a novice question about LINQ, but assuming I have a set of Items with a DateTime property, one date having at most one item, how would I go about selecting the N most recent items from a date of reference, that is, the N items which have a date smaller that the requested date, and the largest date?
My naive thought would be to first select items with a date smaller than the reference date, sort by date, and select the N first items from that subset.

 var recentItems = from item in dataContext.Items
              where item.Date<=date 
              orderby item.Date descending 
              select item;

 var mostRecentItems = recentItems.Take(5).ToList();

Is this the "right" way to do it, or are there obviously better ways to achieve my goal?

Answer 1

Yes, that is the correct method. Judging by the word dataContext in there, I'm assuming that this is Linq to SQL code; it will get turned into an efficient TOP N query.

(Well, "efficient" as long as the data is indexed by date.)

One thing I might change is to remove the ToList(). Unless you actually need it to be a list, it is typically better to leave it alone as an IEnumerable<T>, especially if you just need to iterate over it as opposed to obtaining elements by index.

Edit: I should qualify what I mean by better in reference to ToList. When you call ToList, you end up with an in-memory structure that contains all of the elements in the set. This uses up memory that you don't always need to be using. Creating the list also requires a full iteration of all records, so if you later iterate the list itself, you've gone through every element twice.

For only 5 elements, the difference probably will not be noticeable; however, for 5000 elements it may matter quite a lot. Therefore, you should get in the habit of not using ToList() unless you are certain that you need it. Most of the time, you don't.

Answer 2

I think this is fine. You could also do this in 1 sentence:

 var recentItems = (from item in dataContext.Items
              where item.Date<=date 
              orderby item.Date descending 
              select item).Take(5).ToList();

but your way is equally good.

Answer 3

yeah that's exactly how to do it.