Return IQueryable containing 2 Subclasses

Question

Can you return IQueryable which is composed of two or more different subclasses ? Here's my attempt to show what I mean. It throws the error:

System.NotSupportedException: Types in Union or Concat have members assigned in different order..

var a = from oi in db.OrderItems
        where oi.OrderID == ID
            && oi.ListingID != null
        select new OrderItemA {
            // etc
        } as OrderItem;

var b = from oi in db.OrderItems
        where oi.OrderID == ID
            && oi.AdID != null
        select new OrderItemB {
            //etc
        } as OrderItem;

return a.Concat<OrderItem>(b);

Answer 1

Try doing the concat on IEnumerable instead of IQueryable:

return a.AsEnumerable().Concat(b.AsEnumerable());

If you need an IQueryable result you could do this:

return a.AsEnumerable().Concat(b.AsEnumerable()).AsQueryable();

Doing this will force the concat to happen in-memory instead of in SQL, and any additional operations will also happen in-memory (LINQ To Objects).

However, unlike the .ToList() example, the execution should still be deferred (making your data lazy loaded).