I can't save session with DropDownList, when I select Item I want save item this. I learning mvc 4

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MVC

I there proplem with save session DropDownList

Quốc gia
Tỉnh/thành
Quận/huyện

Example:

In view :
@Html.Action("QUOCGIA","DropDownList", new { Id="TENQUOCGIA" })

@Html.DropDownList("TENQUOCGIA", new SelectList(Model,"TENQUOCGIA","TENQUOCGIA"), "-- Chọn --", new { @class="inputthanhtoan", id=ViewBag.keyquocgia })

In Controller:

[ChildActionOnly]
       public ActionResult QUOCGIA(string Id, DropDownList TENQUOCGIA)  // phai de DropDownList o day nó mới lưu
       {
           var model = db.QUOCGIAs;
           ViewBag.keyquocgia = Id;

           return View(model);
       }

I save session with ajax:

$(function () {
      $("#buttunSessionDangKy").on("click", function () {
          var TAIKHOAN = $("#TAIKHOAN").val();
          var HOTEN = $("#HOTEN").val();
          var CMND = $("#CMND").val();
          var DIACHI = $("#DIACHI").val();
          var SDT = $("#SDT").val();
          var TENQUOCGIA = $("#TENQUOCGIA").val();
          var TINH = $("#TINH").val();
          var TENHUYEN = $("#TENHUYEN").val();

          $.post("@Url.Action("SaveSessionDangKy", "Account")", { TAIKHOAN: TAIKHOAN, HOTEN: HOTEN, CMND: CMND, DIACHI: DIACHI, SDT: SDT, TENQUOCGIA: TENQUOCGIA, TINH: TINH, TENHUYEN: TENHUYEN }, function (data) {
                  // alert("luu session!");
                  return true;
              });
          });
   });

and :

[HttpPost]
        public ActionResult SaveSessionDangKy(THANHVIEN model)
        {
            // Luu vao session thanh vien
            Session["ThanhVien"] = model;
            return Content("");
        }

Thanh you for answer!