Adding methods to LINQ queries

Question

I am trying to create a simple LINQ query like this:

var query = from o in entities.Orders   
        select new Order
        {
            Firstname = o.FirstName,
            Lastname = GetName()
        };

Here I want FirstName to be obtained from database, and I want to supply the Lastname from another source. When I run this I get an exception: "LINQ to Entities does not recognize the method 'System.String GetName()' method, and this method cannot be translated into a store expression."

How can I add my own methods to a LINQ query?

user743382 · Accepted Answer · 2012-07-11 10:09:02Z

2

You cannot, you have to solve the problem you're having some other way, usually by splitting into two projections, one that might get translated to SQL and run on the database server, one that runs on the client.

var query =
    from o in
        (from o in entities.Orders
         select new
         {
             o.FirstName,
         }).AsEnumerable()
    select new Order
    {
        Firstname = o.FirstName,
        Lastname = GetName()
    };

Note: the result is not an IQueryable, and any filtering or sorting that you might add will run on the client too.

answered Jul 11, 2012 at 10:09

user743382

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2 Comments

gunnerz Over a year ago

Thanks hvd. Is there a way to avoid rewritting the other parameters? I have got quite a few parameters along with FirstName. I do not want to repeat them in both the query and the enumeration.

user743382 Over a year ago

You might be able to do something like

var orders = (from o in entities.Orders select new Order { FirstName = o.FirstName }).AsEnumerable().Select(o => { o.Lastname = GetName(); return o; })

, but whether that works depends on how exactly your classes behave. That second Select cannot be written with LINQ syntax, as far as I know.

BenMorel · Accepted Answer · 2013-12-07 19:32:57Z

1

You cannot add your own methods to Linq to entities query because that query is translated to SQL and executed on SQL server. The only allowed custom methods must be either mapped SQL functions or model defined functions (ESQL functions defined inside EDMX).

If you want to execute .NET method in Linq query you must first materialize result (execute SQL) from Linq-to-entities query and continue with Linq-to-objects query executed in your application.

edited Dec 7, 2013 at 19:32

BenMorel

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answered Jul 11, 2012 at 10:08

Ladislav Mrnka

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