Convert new object creation to linq expression for linq to entities

Question

I am doing a query using linq to entities and I want to reuse the same code but with a slightly different select statement. This is the query:

return from f in GetFields<T>()
     where f.Id == Id
     select new Prop<V>
         {
           Name = f.Name,
           Value = toValue(f.value)
         };

where toValue is a Func that creates a new Value object. I want to use several toValue function which have this kind of body:

var toValue = f => new DerivedValueClass {
                 FirstField = f.FirstField,
                 SecondField = f.SecondField
              }

So it is just simple assignment, which will be easily translated by sql to entities. However when I execute the code, linq to entities states:

The LINQ expression node type 'Invoke' is not supported in LINQ to Entities

I guess it is not possible to call the toValue function, since it can't be translated. How can I create an expression from the function toValue in order to use it inside the linq query?

Clynamen Clynamen · Accepted Answer · 2014-06-27T13:24:03

Eventually I find out that Linqkit allows to do this kind of things very easily:

Expression<Func<T, V>> toValue =  a => new DerivedObject() { 
    // perform assignment here
}

return from f in GetFields<T>().AsExpandable() // <- LinqKit AsExpandable() extension method
 where f.Id == Id
 select new Prop<V>
     {
       Name = f.Name,
       Value = toValue.Invoke(f.value) // <- LinqKit Invoke() extension method
     };

It is really important that toValue is an Expression, since the compiler will create an ExpressionTree instead of simply generating a lambda expression. Linqkit use some trick in order to replace the toValue invocation with an expression tree. More info on the LinqKit site and here

Convert new object creation to linq expression for linq to entities

1 Answers