Only parameterless constructors and initializers are supported in LINQ to Entities

Question

I have this error in this linq expression :

var naleznosci = (from nalTmp in db.Naleznosci
                              where nalTmp.idDziecko == idDziec
           


        

Answer 1

yeh, try it like this....

var naleznosci = (from nalTmp in db.Naleznosci
                              where nalTmp.idDziecko == idDziec
                              select new Payments()
                              {
                                  Dziecko.Imie,
                                  Dziecko.Nazwisko,
                                  Miesiace.Nazwa,
                                  Kwota,
                                  RodzajeOplat.NazwaRodzajuOplaty,
                                  RodzajeOplat.TypyOplat.NazwaTypuOplaty,
                                  DataRozliczenia,
                                  TerminPlatnosci
                              }).ToList();

this will new up your Payment object using a parameterless constructor, and then initialize the properties that are listed inside the curly braces { }

Answer 2

Just ToList() the DbSet before the Select statement.. the actual DbSet is saved as a query, it's not fulfilled yet. After calling ToList() you're playing with objects, and then you can use a non-default constructor in the query.

Not the most efficient way usage-time wise, but it's an option on small sets.

Answer 3

Although it is late to answer, it could still help someone in distress. Since LINQ to entities do not support the parameter-less object constructions. However, the projection methods for IEnumerable.

So prior to selection, just convert your IQueryable to IEnumerable by using this code:

var result = myContext.SomeModelClass.AsEnumerable().Select(m => m.ToString());

It will work fine. However, it will, of course, loose the benefits of native queries.

Answer 4

Also, if you want to use a constructor with multiple objects to initialize, you might get error if no values are returned by Linq.

So you might want to do something like this:

(from x in table_1
   join y in table_2
   on x.id equals y.id
   select new {
   val1 = x,
   val2 = y
})
.DefaultIfEmpty()
.ToList()
.Select(a => new Val_Constructor(a.val1 != null ? a.val1 : new Val_1_Constructor(),
                            a.val2 != null ? a.val2 : new Val_2_Constructor()))
.ToList();

Answer 5

If you're like me and don't want to have to populate your properties for each query you're building, there is another way to solve this issue.

var query = from orderDetail in context.OrderDetails
            join order in context.Orders on order.OrderId equals orderDetail.orderId
            select new { order, orderDetail };

At this point you have an IQueryable containing an anonymous object. If you want to populate your custom object with a constructor you can simply do something like this:

return query.ToList().Select(r => new OrderDetails(r.order, r.orderDetail));

Now your custom object (which takes two objects as a parameter) can populate your properties as needed.

Answer 6

First I would avoid the solution with

from ....
select new Payments
{
  Imie = nalTmp.Dziecko.Imie,
  ....
}

This requires an empty constructor and ignores encapsulation so you are saying new Payments() is a valid payment without any data, but instead the object must have at least a value and probably other required fields depending on your domain.

It's better to have a constructor for required fields but only bring needed data:

from ....
select new
{
  Imie = nalTmp.Dziecko.Imie,
  Nazwisko = nalTmp.Dziecko.Nazwisko
  ....
}
.ToList() // Here comes transfer to LINQ to Collections.
.Select(nalImp => new Payments
 (
  nalTmp.Imie,//assume this is a required field
  ...........
  )
  {
     Nazwisko = nalTmp.Nazwisko //optional field
  })
.ToList();