IEnumerable<T> VS IList<T> VS IQueryable<T>

Question

New to the MVC.net scene (and .net for that matter), but seems I find a wide array of options when wanting to populate a "list" with data. In my case at the moment, I'd like to populate a list from a select query of items and render the results in JSON for output, so bear with me....

So, my viewmodel class is something like :

[Serializable()]
public class TFSquery 
{
    public int MsgUid { get; set; }
    public DateTime CreateStamp { get; set; }    
}

And then I'd like to populate it with my query output:

List<TFSquery> z = (from msg in _DB.Msg 
                    select new { msg.MsgUID, msg.CreateStamp }).ToList();

Then would I loop the output into my List so that I can then output in my Json return string? And when I use a LIST VS IENUMERABLE VS IQUERYABLE??

return Json(new { Result = z }, JsonRequestBehavior.AllowGet);     

Answer 1

Each interface has its own set of uses.

IQueryable is for deferred queries (that is, it saves the query but only executes it when it is enumerated)

IEnumerable can be enumerated and that's it.

IList can have items added and removed.

Answer 2

My rules of thumb:

-Use a List when you have to add, remove, or refer to an item by index

-Use a IQueryable when you have to run ad-hoc queries against it

-Use IEnumerable by default

It looks like you're already doing the query "in" your database, so I'd suggest using the simplest version: IEnumerable to simply be able to loop through the results.

Answer 3

If your new to .NET and C# I'd spend some time researching and becoming knowledgeable about what the different collection types are, how they differ, and when to use them. You'll use collections so often it you cannot afford to have a "simple one liner" summary understanding like the other answerers posted.

Here is a good guide on .NET collection types: http://msdn.microsoft.com/en-us/library/0ytkdh4s.aspx

IQueryable is its own special beast and deserves its own guide: http://msdn.microsoft.com/en-us/library/system.linq.iqueryable.aspx

Answer 4

The difference between IEnumerable and IQueryable that deserves to be highlighted is that an IQueryable object executes its underlying LINQ query as it's accessed. So if you pass an IQueryable around your application, you may be hitting the underlying data source more often than you expect.

Once you've retrieved and manipulated the data you need as an IQueryable, I recommend you return it as an IEnumerable, using, for example, IQueryable.ToList().

As has been mentioned, a List is just a particular implementation of IEnumerable; one that allows its items to be inserted/removed/referenced by index. Compare to, say, a Dictionary, which is also an IEnumerable, but is based around a hash table; its items are represented by key-value pairs.