Only parameterless constructors and initializers a

Question

I have this error in this linq expression :

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

Any idea how solve this problem? I try with any combination of expression... :/

Answer 1

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();

Answer 2

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 3

If you still want to use your constructor for initialization and not properties (sometimes this behaviour is desired for initialization purposes), enumerate the query by calling ToList() or ToArray(), and then use Select(…). Thus it will use LINQ to Collections and that limitation of not being able to call constructor with parameters in Select(…) will vanish.

So your code should look something like this:

var naleznosci = db.Naleznosci
                          .Where(nalTmp => nalTmp.idDziecko == idDziec)
                          .ToList() // Here comes transfer to LINQ to Collections.
                          .Select(nalImp => new Payments
                              (
                                  nalTmp.Dziecko.Imie,
                                  nalTmp.Dziecko.Nazwisko,
                                  nalTmp.Miesiace.Nazwa,
                                  nalTmp.Kwota,
                                  nalTmp.RodzajeOplat.NazwaRodzajuOplaty,
                                  nalTmp.RodzajeOplat.TypyOplat.NazwaTypuOplaty,
                                  nalTmp.DataRozliczenia,
                                  nalTmp.TerminPlatnosci
                              ))
                          .ToList();

Answer 4

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 5

Having just encountered this error myself, I thought I would add that if the Payment type is a struct, you would also encounter the same error because struct types do not support parameterless constructors.

In that event, converting Payment to a class and using the object initializer syntax will resolve the issue.

Answer 6

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.