Casting the results of linq-to-sql?

What you're doing is correct (you can't call Cast directly on the IQueryable ), but it's sub-optimal, because you're creating 2 lists when you really only need one. Use AsEnumerable instead of the first ToList : return items.AsEnumerable().Cast().ToList() Calling AsEnumerable will cause Cast to be evaluated on a IEnumerable not IQueryable so the cast will be done on the actual object returned from the DB, rather than directly in the DB.

What you're doing is correct (you can't call Cast directly on the IQueryable), but it's sub-optimal, because you're creating 2 lists when you really only need one. Use AsEnumerable instead of the first ToList : return items.AsEnumerable().Cast().ToList(); Calling AsEnumerable will cause Cast to be evaluated on a IEnumerable, not IQueryable, so the cast will be done on the actual object returned from the DB, rather than directly in the DB.

That's what I was hoping for, to avoid creating both lists. – Kelly Cline Jul 20 '10 at 18:58.

Alternately, you could do this instead: var items = from t in SomeTable where t. Condition == true select (SomeInterface) t; return items.ToList(); If t doesn't implement SomeInterface it will still fail at runtime, though. This will do the same thing, but won't fail; rather it will give you null if the object doesn't implement the interface: var items = from t in SomeTable where t.

Condition == true select t as SomeInterface; return items.ToList().

The question explains the first call to ToList, and there is good reason for it (although it should be AsEnumerable, not ToList). Calling Cast directly on the IQueryable would cause it to be executed on the DB, which can't work – Thomas Levesque Jul 20 '10 at 17:36.

A few solutions... Instead of converting data after collecting it, you could consider make a projection while collecting it. Return items. Select(x=>x as SomeInterface).ToList(); If you do this frequently, an extension method may still be helpful.It doesn't have to be Tolist/Cast/ToList as you mentioned though.

Cast will take an IEnumerable already, but you can create an overload that accepts an IQueryable, something like this (untested). Public IEnumerable Cast(this IQueryable input) { return input.AsEnumerable().Cast(); } This would whittle your code down to this: return items.Cast.ToList().

We were close! But since there is a cast involved, I needed a and a . Thanks!

– Kelly Cline Jul 20 '10 at 22:25 Yep, you're right. Edited the code to reflect this. – Phil Gilmore Jul 22 '10 at 22:06.

Try following public IQueryable fGetNewsTags(int id) { var result = from news in context.GetTable() where news. M_Id == id select (ITag)news; return result; }.

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