Using IQueryable Or IEnumerable inside my search action method - asp.net-mvc-3

Question

i have the following action method:-
[AcceptVerbs(HttpVerbs.Post)]
public PartialViewResult Search(string q, int classid)
{
var users = r.searchusers(q, classid);
// code does here..............
which calls the following model repository method:-
public IQueryable<User> searchusers(string q, int id)
{
return from u in entities1.Users
where (!u.Users_Classes.Any(c => c.ClassID == id) && (u.UserID.Contains(q))
select u;
}
now if i change the IQueryable to IEnumerable as follow , will there be any changes on how the query will be executed in this case ?:-
public IEnumerable<User> searchusers(string q, int id)
{
return from u in entities1.Users
where (!u.Users_Classes.Any(c => c.ClassID == id) && (u.UserID.Contains(q))
select u;
}

Answer 1

Yes, in my testing once you cast to IEnumerable, that determines the query SQL. Any additional query composition you do after that will be done in memory after the query is executed.
So suppose you have a base query that loads a list of users and returns an IEnumerable. Then before you actually run through that list (thereby executing the query), you also add a .Where(i=>i.username='bob'). In that case, it will execute the whole select, and then apply a LINQ-to-Objects in memory filter for the "where username='bob'" part, which is probably not what you want, instead you want the whole thing to be run as part of the SQL statement.
So yes, always use IQueryable whenever you can so that your fully composed are run at once.

Answer 2

Yes it will change the query, you want to use the IQuerable for anything that uses a remote datasource.
In your case it will force linq to execute the query, where as IQuerable would wait until someone else to execute the query. IQuerable allows them, if they desire, to append more conditions to push down to the database for execution.
I generally enforce IEnumerable at the layer boundary, places where I dont want people modifying the queries that the system is going to generate.