Question

My question is really simple. When should I use List, IEnumerable and ArrayList.

Here’s my scenario. I’m working in a Web app using LINQ. Information is returned as a IEnumerable:

IEnumerable<Inventory> result = from Inventory i in db where.... 

I’m not sure how IEnumerable works, but every operation takes a lot of time to execute. More specifically, result.Count(), result.ElementAt(i), result.ToList, etc, each operation takes a considerable amount of time.

So, I was wondering if I should treat this as a List by doing result.ToList, instead of working with the IEnumerable variable.

Thanks!

Answer 1

If I understand what you’re doing correctly, you have a query like from Inventory i in db select i and then you do several operations on the result:

var count = result.Count();
var fifth = result.ElementAt(5);
var allItems = result.ToList();

Now consider what happens when you have the query as different types:

• IQueryable<T>

  var result = from Inventory i in db select i;
  IQueryable<Inventory> result = from Inventory i in db select i;
  

  The two lines above are the same. They don’t actually go to the database, they just create a representation of the query. If you have this, Count() will execute an SQL query like SELECT COUNT(*) FROM Inventory, ElementAt(5) will execute another query that takes only the fifth item in the table and ToList() will execute something like SELECT * FROM Inventory, but that’s what we want here.

• IEnumerable<T>

  IEnumerable<Inventory> result = from Inventory i in db select i;
  

  Doing this again does not go the database, it only creates a representation of the query. But it’s a representation that can’t use the methods specific to IQueryable<T>, so any LINQ operation will enumerate the collection, which will execute an SQL query like SELECT * FROM Inventory.

  So, for the example: Count() will execute the SELECT * … query only to count the items in the result. ElementAt(5) will execute the whole query again, only to throw away all items except for the fifth. And ToList() will execute the query one more time.

• List<T>

  List<Inventory> result = (from Inventory i in db select i).ToList();
  

  This will actually execute the SELECT * FROM Inventory query immediately and once. All operations you do with result won’t touch the database, they will be done in-memory.

What should you take away from this? First, never use IEnumerable<T> as the type of the database query. It has horrible performance.

If you want to make several distinct operations on the result, using IQueryable<T> might be the best solution.

If you want to retrieve the whole result anyway, use ToList() (or ToArray()) as soon as possible and then work with the resulting List<T>.