Filter Data in ASP.Net MVC using Linq query

I want to add Filter on Column (Del_ID is null)

below is code

        public ActionResult AllUserTypes()
        {
            if (string.IsNullOrEmpty(Convert.ToString(Session["UserName"])))
            {
                RedirectToAction("Login", "Home");
            }

            var userid = 0;
            var usertypeid = 0;
            int.TryParse(Convert.ToString(Session["UserID"]), out userid);
            int.TryParse(Convert.ToString(Session["usertypeid"]), out usertypeid);

            if (usertypeid != 1)
            {
                return RedirectToAction("Admin", "DashBoard");
            }

            var List = new List<UserTypeMV>();

            var usertypes = DB.tblUserTypes.ToList();
            foreach (var Usertype in usertypes)
            {
                List.Add(new UserTypeMV() { UserTypeID = Usertype.UserTypeID, UserType = Usertype.UserType, Designation = Usertype.Designation,Del_ID=Usertype.Del_ID });
            }

            return View(List);
        }

how to use where clause in above code