Code First Entity Framework, select ViewModel - constructor with parameter

Question

I create class Employee:

public class Employees : BaseModel
{
    [Key]
    public int EmployeeId { get; set; }
   // [ohter parameter]
    public int? DepartmentId { get; set; }
    public virtual Departments Departments { get; set; }
    public int? OccupationId { get; set; }
    public virtual Occupations Occupations { get; set; }
}

I want to create a view model.

public class Employee_ViewModel 
{
    public Employee_ViewModel(Employees item)
    {
        this.EmployeeId = item.EmployeeId;
        //[other parameter]
        this.DepartmentId = item.DepartmentId;
        this.OccupationId = item.OccupationId;
    }

    public int EmployeeId { get; set; }
    //[other parameter]
    public string DepartmentName { get; set; }
    public string OccupationName { get; set; }
}

my method which returns Employee_ViewModel look like:

 List<Employee_ViewModel> item = contex.Employees.Where(w => w.IsActive == true && w.IsVisible == true)
                                     .Include(i => i.Departments)
                                     .Include(i => i.Occupations)
                                     .Select(s => new Employee_ViewModel(s)
                                     {
                                         OccupationName = s.Occupations.Name,
                                         DepartmentName = s.Departments.Name
                                     })
                                     .ToList();

Finally I get error:

Only parameterless constructors and initializers are supported in LINQ to Entities.

Solution form the this post:

Only parameterless constructors and initializers are supported in LINQ to Entities

is working but why my constructor doesn't work?

Isn't it obvious from the exception message (the word parameterless). Also this has been asked many times stackoverflow.com/…. — Ivan Stoev
– Ivan Stoev, Commented Jan 15, 2017 at 21:26

Matías Fidemraizer · Accepted Answer · 2017-01-15 22:14:29Z

2

As other tools, Entity Framework creates proxies of your models on run-time, and these are emitted as derived classes of your models. Clearly, you can't expect a proxy generator to use a constructor with parameters to instantiate your model.

In summary: keep it simple, use the tool the way it's meant to be used: your entities should contain at least their parameterless constructor.

Consider using AutoMapper to keep things simple:

// If you go for the AutoMapper approach, you need no constructor 
// with parameters or factory method anymore!
List<Employee_ViewModel> item = contex.Employees.Where(w => w.IsActive == true && w.IsVisible == true)
                                     .Include(i => i.Departments)
                                     .Include(i => i.Occupations)
                                     .ProjectTo<EmployeeViewModel>()
                                     .ToList();

See AutoMapper's docs to queryable extensions to learn more about how to combine it with Entity Framework.

edited Jan 15, 2017 at 22:14

answered Jan 15, 2017 at 20:57

Matías Fidemraizer

65.1k19 gold badges132 silver badges216 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

18666 Over a year ago

I used solution with functions From, but return error: LINQ to Entities does not recognize the method 'MS.ViewModel.Employee_ViewModel From(MS.Models.Employees)' method, and this method cannot be translated into a store expression. What about performance, better use AutoMapper?

18666 Over a year ago

Adding ToList() before Select solving a problem. But is correct solution?

Matías Fidemraizer Over a year ago

@18666 My first attemp was wrong. You're right, expression trees should be translatable to SQL and a custom factory isn't possible to be translated into SQL. See my updated answer on which I've thrown away the wrong part, and I've edited the AutoMapper approach.

Matías Fidemraizer Over a year ago

@18666 As of your attemp of calling ToList and later Select, it's not a good idea, because usually you don't want to get your entire table in memory to do filters, projections and so on. Probably you want them to happen in the database.

Collectives™ on Stack Overflow

Code First Entity Framework, select ViewModel - constructor with parameter

1 Answer 1

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related