Linq to sql performance

I'm creating a system and I'm using EF with Linq. Model First.

So I create my Model (.edmx) and using it, I generated my database and my classes. like the Usuario (user in portugues. going to keep the names in portuguese to avoid some misleading)

namespace SistemaBox.Model
{
    public partial class Usuario
    {
        public int Codigo { get; set; }
        public string Nome { get; set; }
        public string Sobrenome { get; set; }
        public string Senha { get; set; }
        public DateTime DataCriacao { get; set; }
        public DateTime DataUltimoLogin { get; set; }

        public virtual PermissaoGrupo CodigoPermissaoGrupo { get; set; }
    }
}

After this, I create my controller class

namespace SistemaBox.Controller
{
    [Serializable]
    public class ZUsuario : Usuario
    {
        String connString = ConfigurationManager.ConnectionStrings["ConnectionString"].ToString();

        /// <summary>
        /// Cadastro e edição
        /// </summary>
        public void Cadastro()
        {
            if (Codigo != null && Codigo > 0)
            {
                // Cadastro
                using (DataContext db = new DataContext(connString))
                {
                    Table<Usuario> Usuarios = db.GetTable<Usuario>();
                    var query =
                        from usr in Usuarios
                        where usr.Nome == "Test"
                        select usr;

                    foreach (var usr in query)
                        Console.WriteLine("id = {0}, City = {1}", usr.Codigo, usr.Nome);
                }
            }
            else
            {
                // Edição

            }
        }
    }
}

PS: Please ignore the fact that the class name is record and I'm doing a select. just for tests.

My question is about the controller.

Is the right way to declare the connection String as global

String connString = ConfigurationManager.ConnectionStrings["ConnectionString"].ToString();

and use the using like I'm doing?