How to force to user for login ?

[HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult login(reshabregi u)
        {
            if(ModelState.IsValid)
            {
                var v = db1.reshabregis.Where(a => a.Username1.Equals(u.Username1) && a.Password1.Equals(u.Password1)).FirstOrDefault();
                if(v != null)
                {
                    Session["username"] = u.Username1;
                    return RedirectToAction("Index", "uploadfile");
                }
               else
                {
                    ViewBag.message = "ENter username and password properly";
                }
            }
            return View();
        }