Initializing a non-entity ref type in select creates a single instance for every record

[kszicsillag]

Consider the following entity type and its DTO type.

// Entity
public class Dog
{
    public int Id { get; set; }
    public string Name { get; set; }
}

//DTO
 public class DogDTO
 {
     public int ID { get; set; }
     public string  Name { get; set; }
     public IDCard IDCard { get; set; }
 }

 public class IDCard
 {
     public int ID { get; set; }
     public string Number { get; set; }
 }

Now query all the entities and project them to the DTO type.

 var dogDTOList = ctx.Dogs.Select(
   d => 
     new DogDTO { ID = d.Id
    ,Name = d.Name
    ,IDCard = new IDCard() })
.ToList();

We would expect that a new IDCard instance will be created for each DogDTO - as this is what would happen in case of LINQ-to-Objects.
However, the constructor of IDCard is called only once and the IDCard instance will be the same for all DogDTOs.

There is a full repro repo here.

Further technical details

EF Core version: 2.1.0
Database Provider: Microsoft.EntityFrameworkCore.SqlServer
Operating system: Windows Server 2016
IDE: Visual Studio 2017 15.7

[kszicsillag]

We observed that if the constructor is called with different parameter values for every record (e.g. with some data from the record) we get the expected beahvior: different instance created for every record.

[smitpatel]

Duplicate of #7983

[smitpatel]

IDCard does not depend on the range variable hence being evaluated beforehand causing it to have same object for each result. There is no easy way to figure out this case.

[kszicsillag]

Thanks for pointing me to #7983.
If this won't be fixed, at very least the documentation should warn about this issue, clearly state the limitations and give guidelines about how to write a projection without falling into this kind of pitfall.

[ajcvickers]

Note from triage: we will reconsider this, perhaps pivoting on whether or no the call to new is in the final projection. Also, Relinq makes this problematic because it erases the New expression.

[smitpatel]

We have done work to now create parameter for NewExpression query tree. So this should likely be fixed.