Question

What I’m trying to do is to create a custom type, with a custom attribute that will store both Id and Name from a record. (Something like “223 – Robert Smith”). This is what I’m doing:

return (from c in db.Credores
        where c.Status == true
        select new CredorCompleto
        {
            Id = c.Id,
            Nome = c.Nome,
            NomeCompleto = c.Id + c.Nome,
            CNPJ = c.CNPJ
        }).ToList();

Update: Definition for ‘CredorCompleto’

public class CredorCompleto
{
    public string NomeCompleto { get; set; }
    public string CNPJ { get; set; }
    public string Nome { get; set; }
    public int Id { get; set; }
}

This is what I’m getting:

Unable to cast the type System.Int32 to type System.Object. LINQ to Entities only supports casting Entity Data Model primitive types.

Answer 1

Your comment on @Moon’s answer provides an important clue:

“LINQ to Entities does not recognize the method System.String Format(System.String, System.Object, System.Object) method, and this method cannot be translated into a store expression.”

The problem might be that db.Credores is an IQueryable, and not just an IEnumerable. So when your LINQ to SQL provider tries to analyse your original query, it comes upon a bit that it does not recognise, and does not know how to translate to a SQL query.

I suppose the LINQ to SQL provider has problems converting your concatenation c.Id + c.Nome into a valid SQL statement, possibly because the former is an int and the latter a string.

What’s for sure is that it definitely doesn’t know how to transform a call to string.Format() to SQL (which is not surprising, since SQL doesn’t have that function).

So you could try to execute the SQL query before you perform .NET-specific logic on it. Try this:

return db
       .Credores
       .Where(c => c.Status == true)
       .AsEnumerable() // <-- this should trigger the execution of the SQL query
       .ToList()       // <-- and if it does not, then this certainly will
       .Select(c => new CredorCompleto
                    {
                        …
                    })
       .ToList();

The call to .AsEnumerable() — and a call to .ToList() is probably required also, IIRC — will trigger the execution of the SQL query. Everything after that operates on an in-memory IEnumerable, not on a IQueryable. This means that after the .ToList(), LINQ will do no more smart code analysis, or attempt to transform the remaining operators to SQL.