including "offline" code in compiled querys

Question

What happens behind the curtains when I include a function into my compiled query, like I do with DataConvert.ToThema() here to convert a table object into my custom business object:

public static class Queries
{
    public static Func<MyDataContext, string, Thema> GetThemaByTitle
    {
        get
        {
            var func = CompiledQuery.Compile(
                (MyDataContext db, string title) =>
                    (from th in elan.tbl_Thema
                     where th.Titel == title
                     select DataConvert.ToThema(th)).Single()
                     );
            return func;
        }
    }
}

public static class DataConvert
{
    public static Thema ToThema(tbl_Thema tblThema)
    {
        Thema thema = new Thema();

        thema.ID = tblThema.ThemaID;
        thema.Titel = tblThema.Titel;
        // and some other stuff

        return thema;
    }
}

and call it like this

Thema th = Queries.GetThemaByTitle.Invoke(db, "someTitle");

Apparently the function is not translated in to SQL or something (how could it), but it also does not hold when I set a breakpoint there in VS2010.

It works without problems, but I don't understand how or why. What exactly happens there?

Answer 1

Your DataConvert.ToThema() static method is simply creating an instance of a type which has a default constructor, and setting various properties, is that correct? If so, it's not terribly different from:

(from th in elan.tbl_Thema
where th.Titel == title
select new Thema{ID=th.ThemaID, Titel=th.Titel, etc...}
).Single());

When you call Queries.GetThemaByTitle, a query is being compiled. (The way you are calling this, by the way, may or may not actually be giving you any benefits from pre-compiling). That 'Query' is actually a code expression tree, only part of which is intended to generate SQL code that is sent to the database.

Other parts of it will generate IL code which is grabbing what is returned from the database and putting it into some form for your consumption. LINQ (EF or L2S) is smart enough to be able to take your static method call and generate the IL from it to do what you want - and maybe it's doing so with an internal delegate or some such. But ultimately, it doesn't need to be (much) different from what would be generated from I substituted above.

But note that this happens regardless what the type is that you get back; somewhere, IL code is being generated that puts DB values into a CLR object. That is the other part of those expression trees.

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If you want a more detailed look at those expression trees and what they involved, I'd have to dig for ya, but I'm not sure from your question if that's what you are looking for.