Entity framework core 2.1: could not be translated and will be evaluated locally

Question

A simple query:

(from user in _db.Users
select new UserObj
{
   DisplayName = user.LastName + " " + user.FirstName
}).OrderBy(o => o.DisplayName)
  .Where(w => w.EndDate == null)
  .ToList();

The model class is:

public class UserObj
{
    public string FirstName {get; set;}
    public string LastName {get; set;}
    public string DisplayName {get; set;}
    public DateTime? EndDate {get; set;}
 }

However I got the warning

warn: Microsoft.EntityFrameworkCore.Query[20500] The LINQ expression 'where(new UserObj() {DisplayName = (([user].LastName + " " + [user.FirstName)}.EndDate == null)' could not be translated and will be evaluated locally.

Answer 1

Order of the statements is important. EF Core will only allow non-translatable expressions (*) , such as the string concatenation, in the last select statement.

So,

• either reorganize your code so that the select statement comes last (before ToList() )
• or add a computed column DisplayName in your database which equals to LastName + ' ' + FirstName and query this column in your custom DTO object.

(*) things that EF Core not yet is able to translate into a similar server-side statement for the different data providers

Answer 2

Thanks for the comment by @NetMage. EndDate should be initialized. Added it the error is gone.

(from user in _db.Users
 select new UserObj
 {
     DisplayName = user.LastName + " " + user.FirstName,
     EndDate = user.EndDate
  }).OrderBy(o => o.DisplayName)
  .Where(w => w.EndDate == null)
  .ToList();