Question

If I wanted to return just the ID, and Name from a collection of Customer entities, what would be the recommended way of doing this? Should I use the Entity Set and then pick off what I need as I loop the collection? Is there LINQ syntax that can give me the equivalent of a SQL Select clause (i.e. Select ID, Name From Customer).

Thank you,
Stephen

Answer 1

Use Queryable.Select<TSource, TResult> to project the collection:

var customers = ctx
   .Customers
   .Where(x => x.FirstName == "Joe")
   .Select(x => new { ID = x.CustomerId, Name = x.CustomerName })
   .ToList();

Note that will project the results into a List<T> of anonymous types.

If you want to project it into something else (e.g custom class/POCO), you’ll need to materalize the result set first, and then project the query (e.g .ToList() and then .Select()).

In other words, if you do this:

var customers = ctx
       .Customers
       .Where(x => x.FirstName == "Joe")
       .Select(x => new CutDownCustomer { ID = x.CustomerId, Name = x.CustomerName })
       .ToList(); // fire query

You will get an EF error (cannot be translated to Linq-Entities query – as “CutDownCustomer” is not part of the conceptual model).

So you must do this:

var customers = ctx
       .Customers
       .Where(x => x.FirstName == "Joe")
       .ToList() // fire query
       .Select(x => new CutDownCustomer { ID = x.CustomerId, Name = x.CustomerName });

Of course, if you only require method scope for your result set, anonymous types should be sufficient.

HTH.