Linq to Entities - Projections against Query Syntax vs Method Syntax

Question

Since LINQ query expression are translated "under the covers" to call the same methods that a corresponding method query would call (at least I think so), I would expect these two queries to return the same type. For some reason though this:

var result = from i in db.Invoices
             select new { i.InvoiceNum };

sets result as an IQueryable<'a> with each member having an InvoiceNum property, while this

IQueryable<string> result2 = db.Invoices.Select(i => i.InvoiceNum);

Is smart enough to return IQueryable<string> (obviously, since that compiles)

Clearly one of my assumptions is wrong, and I was hoping an expert could help me understand a bit better.

(this is EF4, but the same happens with linq-to-objects, and I'm guessing the same would also happen with L2S)

Answer 1

When you write new { } you are creating an anonymous type

try

var result = from i in db.Invoices
             select i.InvoiceNum;

and you will see that it returns the type you expect.

Answer 2

those are not the same, the first one is returning anonymousn type, to make them the same you need to have th first one as:

var result = from i in db.Invoices
         select i.InvoiceNum;

Answer 3

In your first statement, you create an anonymous type with one property named "InvoiceNum". This happens because you use the new { } syntax. That anonymous type is not a String. The equivalent method syntax would be:

var result = db.Invoices.Select(i => new { i.InvoiceNum });