Question

I was looking up an example how to use a particular part of the Telerik grid framework (ASP.NET MVC3, but that’s not really relevant here). They have a chunk of code which takes a list of filter descriptions and builds an expression:

System.Linq.Expressions.Expression<Func<MyModel, bool> exp = 
               ExpressionBuilder.Expression<MyModel>(listOfFilters);

Ok, so I think that’s fine. The Expression wraps a lambda, which operates on MyModel to produce a bool. Great. Now, their example simply drops that into a Where like so:

someList = someList.Where(exp);

Which I am presuming should be “hey apply that expression to all the items in the list (which is of course generic MyModel too). However, VS claims that code doesn’t compile. I get “No overload Where exists or System.Func has some invalid arguments”.

I played around with it and find that I can compile the expression, which gives the more unseemly looking

someList = someList.Where(x => exp.Compile()(x));

Which does compile and would probably work, but it makes me uncomfortable because I’m now clearly operating outside what I know.

Is there some reason why (IDE setting, flag, stale documentation) the example’s approach doesn’t work?
Is there a rough equivalency to my hack and the example?
Should I structure that hack differently to avoid some hideous problem (like, its not going to compile the expression each and every time it checks an item in the list, right? I think it’s smart enough for that?)

— Edit
Yes it was IEnumerable. I fell into the “all Wheres are created equal” trap.
Thanks all!

Answer 1

Have you tried just doing

someList = someList.Where(exp.Compile());

If someList is an IEnumerable you will need to use exp.Compile() which returns a Func<MyModel,bool>

If someList is an IQueryable you can use exp.Compile() or you can use exp which is an Expression<Func<MyModel, bool>>