public ActionResult ThemSP(int IDSP, int soLuong)
        {
            int IDBan = int.Parse(Session["IDBAN"].ToString());

            ganSession(IDBan);
            List <banDAO> giohang = Session["banDAO"] as List <banDAO>;  // Gán qua biến giohang dễ code
            int           donGia  = (int)db.Sphams.Find(IDSP).Gia;

            if (giohang.FirstOrDefault(t => t.IDSanPham == IDSP) == null)
            {
                banDAO ban = new banDAO()
                {
                    IDBan     = IDBan,
                    IDSanPham = IDSP,
                    DonGia    = donGia,
                    SoLuong   = soLuong,
                    TenSP     = db.Sphams.Find(IDSP).TenSanPham
                };
                giohang.Add(ban);
                if (giohang.Count == 1)
                {
                    HoaDon additemHD = new HoaDon()
                    {
                        GioVao    = DateTime.Now,
                        MaBan     = IDBan,
                        TinhTrang = false,
                        TongTien  = donGia * soLuong
                    };
                    db.HoaDons.Add(additemHD);
                    db.SaveChanges();
                    db.Bans.Find(IDBan).TinhTrang = 0;
                }
                else
                {
                    HoaDon hoaDon = db.HoaDons.FirstOrDefault(t => t.MaBan == IDBan && t.TinhTrang == false);
                    hoaDon.TongTien += donGia * soLuong;
                }
                int  maHD        = (int)db.HoaDons.FirstOrDefault(t => t.MaBan == IDBan && t.TinhTrang == false).MaHD;
                CTHD additemCTHD = new CTHD()
                {
                    MaHD      = maHD,
                    MaSP      = IDSP,
                    SoLuong   = soLuong,
                    ThanhTien = soLuong * donGia
                };
                db.CTHDs.Add(additemCTHD);
                db.SaveChanges();
            }
            else
            {
                banDAO ban = giohang.FirstOrDefault(t => t.IDSanPham == IDSP);
                ban.SoLuong += soLuong;
                int maHD = (int)db.HoaDons.FirstOrDefault(t => t.MaBan == IDBan && t.TinhTrang == false).MaHD;
                db.CTHDs.FirstOrDefault(t => t.MaHD == maHD && t.MaSP == IDSP).SoLuong   += soLuong;
                db.CTHDs.FirstOrDefault(t => t.MaHD == maHD && t.MaSP == IDSP).ThanhTien += soLuong * donGia;
                db.HoaDons.Find(maHD).TongTien += soLuong * donGia;
                db.SaveChanges();
            }
            return(Redirect(Request.UrlReferrer.ToString()));
        }
        public void ganSession(int IDBan)
        {
            Session["banDAO"] = new List <banDAO>(); // khoi tao session ban tam aa
            List <banDAO> temp = Session["banDAO"] as List <banDAO>;
            List <CTHD>   cthd = db.CTHDs.Where(t => t.HoaDon.MaBan == IDBan && t.HoaDon.TinhTrang == false).ToList();

            if (cthd == null)
            {
                return;
            }
            foreach (CTHD item in cthd)
            {
                banDAO ban = new banDAO()
                {
                    IDBan     = (int)IDBan,
                    IDSanPham = item.MaSP,
                    DonGia    = (int)item.Spham.Gia,
                    SoLuong   = (int)item.SoLuong,
                    TenSP     = item.Spham.TenSanPham
                };
                temp.Add(ban);
            }
        }