Linq to SQL construct a custom object externally

Question

I want to achieve the following:

return  (from a in db.Tags
    select new TagItem
    {
        ID = a.Id,
        Name = a.Name
    });

..but i don't want to achieve it like that, because i need to use the exact same TagItem construction elsewhere (for a join)

So this is the intention, to call a method that constructs the TagItem for me:

return (from a in db.Tags
    select ConstructTagItem(a));

And the method constructs the object the same way:

private TagItem ConstructTagItem(Tag a)
{
    return new TagItem { ID = a.Id, Name = a.Name};
}

But this gives me the following error:

Method 'TagItem ConstructTagItem(Tag)' has no supported translation to SQL.

Is there any way to achieve this ?

Solution (thanks Daniel Hilgarth):

return db.Tags.Select(ConstructTagItem);

And the method:

private Expression<Func<Tag, TagItem>> ConstructTagItem
{
    get { return a => new TagItem {ID = a.Id Name = a.Name }; }
}

Daniel Hilgarth · Accepted Answer · 2011-07-20 11:18:41Z

3

Yes, there is. You need to use an expression tree like this:

Expression<Func<Tag, TagItem>> constructTagItem = a => return new TagItem
                                                       { ID = a.Id, 
                                                         Name = a.Name};

And then you can use that expression instead of your method:

return db.Tags.Select(constructTagItem);

According to this article you need to use the method chains way of defining your query.

answered Jul 20, 2011 at 11:18

Daniel Hilgarth

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4 Comments

Frost Over a year ago

I have just figured out i had to use expressions, and then you wrote with this brilliant answer, thanks! ..i just have one question, I will try to explain it in my question, give me a sec

Daniel Hilgarth Over a year ago

@Frost: IMHO, this should be a new question, because it is something else. I don't think it can be done the way your first example can be done. Basically, you just changed your complete question :) Please create a new question and accept my answer here.

Frost Over a year ago

Will do, but i wrote: "because i need to use the exact same TagItem construction elsewhere (for a join)"

Daniel Hilgarth Over a year ago

@Frost: Yeah, I thought your join would also select a TagItem as the end result and not a NewsTagItem.

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