public static bool LoginUsuario(string usr, string pass)
        {
            DataTable dtUser = UserPersistence.GetAllUsuarios();

            var exist = dtUser.Select("NombreUsuario = '" + usr + "' and Clave = '" + pass + "'");

            if (dtUser != null && exist.Count() > 0)
            {
                return(true);
            }
            else
            {
                return(false);
            }
        }