Question

I’ve made a generic extension method that executes an action on an object and returns the object after that:

    public static T Apply<T>(this T subject, Action<T> action)
    {
        action(subject);
        return subject;
    }

I’m unable to use this extension method in an EntityFramework Linq query because of:

An expression tree may not contain an assignment operator

Why is this?

The Linq query:

var parents = from p in context.Parent
               join phr in context.Child on p.key equals phr.parentkey
               into pr
               select p.Apply(
                      x => x.Children = //The assignment operator that fails to build...
                      pr.ToDictionary(y => y.childkey, y => y.childname));

Answer 1

Well, assignment operator aside, how would you expect your Apply method to be translated into SQL? Entity Framework doesn’t know anything about it, and can’t delve into opaque delegates, either.

I suspect what you really need to do is separate out the bits to do in the database from the bits to do locally:

var dbQuery =  from p in context.Parent
               join phr in context.Child on p.key equals phr.parentkey into pr
               select new { p, phr };

var localQuery = dbQuery.AsEnumerable()
                        .Select(pair => /* whatever */);