Question

I was playing around with expression trees and various Linq syntax. I wrote the following:

using (NorthwindDataContext DB = new NorthwindDataContext())
        {
            DataLoadOptions dlo = new DataLoadOptions();

            // Version 1
            dlo.AssociateWith<Customer>(c => c.Orders.Where(o => o.OrderID < 10700).Select(o => o)); 

            // Version 2
            dlo.AssociateWith<Customer>(c => from o in c.Orders
                                             where o.OrderID < 10700
                                             select o);
        }

The Version 1 method returns an error saying “The operator ‘Select’ is not supported in Subquery.”

While Version 2 runs just fine. From what I understand I am writing the exact same thing, but one is with the “dot” notation syntax and the other is query expression syntax.

Am I missing something here? Why the error on one but not the other “if” they are in fact the same query?

Answer 1

To expand on Daniel’s answer, the select o is known as a degenerate query expression – and it’s removed by the C# compiler. So your query is translated to:

c.Orders.Where(o => o.OrderID < 10700)

Note that without the where clause, however, the compiler would still include the Select call, so:

from o in c.Orders
select o

is translated to

c.Orders.Select(o => o)

From section 7.15.2.3 of the language spec:

A degenerate query expression is one
that trivially selects the elements of
the source. A later phase of the
translation removes degenerate queries
introduced by other translation steps
by replacing them with their source.
It is important however to ensure that
the result of a query expression is
never the source object itself, as
that would reveal the type and
identity of the source to the client
of the query. Therefore this step
protects degenerate queries written
directly in source code by explicitly
calling Select on the source. It is
then up to the implementers of Select
and other query operators to ensure
that these methods never return the
source object itself.