listAccounts.Add(new Account { Username = "acc1", Password = "123", Roles = new String[] {"superadmin", "admin", "employee"} });
    listAccounts.Add(new Account { Username = "acc2", Password = "123", Roles = new String[] {"admin", "employee"} });
    listAccounts.Add(new Account { Username = "acc3", Password = "123", Roles = new String[] {"employee"} });

public AccountModel() {
    DataTable dt_usuarios = new DataTable();
    DataTable dt_roles = new DataTable();
    String sql = "SELECT username,password FROM usuarios";
    conexion.conectar();
    MySqlDataAdapter datos = new MySqlDataAdapter(sql, conexion.con);
    datos.Fill(dt_usuarios);
    List<String> roles_list = new List<String>();
    foreach (DataRow user_name in dt_usuarios.Rows)
    {
        String sql_roles = "SELECT r.rol FROM roles r,userroles ur,usuarios u WHERE u.username='" + user_name[0].ToString() + "' AND u.id_usuarios=ur.id_usuarios AND ur.id_roles=r.id_roles";
        MySqlDataAdapter datos_roles = new MySqlDataAdapter(sql_roles, conexion.con);
        datos_roles.Fill(dt_roles);
        foreach (DataRow roles_user in dt_roles.Rows)
        {
            roles_list.Add(roles_user[0].ToString());
        }

        listAccounts.Add(new Account { Username = user_name[0].ToString(), Password = user_name[1].ToString(), Roles = new String[] { roles_list } });
    }
    conexion.cerrar();
    //listAccounts.Add(new Account { Username = "acc1", Password = "123", Roles = new String[] {"superadmin", "admin", "employee"} });
    //listAccounts.Add(new Account { Username = "acc2", Password = "123", Roles = new String[] {"admin", "employee"} });

    //listAccounts.Add(new Account { Username = "acc3", Password = "123", Roles = new String[] {"employee"} });
}