public string LockLichSuGianHang(string id)
        {
            LichSuGianHang lichsugianhang = context.LichSuGianHang.Where(gh => gh.Id == Guid.Parse(id)).SingleOrDefault();

            lichsugianhang.TinhTrang = "Khoá";
            context.SaveChanges();
            return("Khoá thành công");
        }
        public string EditLichSuGianHang(string id, string idgianhang)
        {
            LichSuGianHang lichsugianhang = context.LichSuGianHang.Where(gh => gh.Id == Guid.Parse(id)).SingleOrDefault();
            GianHang       gianhang       = context.GianHang.Where(gh => gh.Id == Guid.Parse(idgianhang)).SingleOrDefault();

            lichsugianhang.IdGianHang  = Guid.Parse(idgianhang);
            lichsugianhang.NgayKetThuc = lichsugianhang.NgayBatDau.Value.AddDays((double)gianhang.ThoiGian);
            context.SaveChanges();
            return("Sửa thành công");
        }
        public string CreateLichSuGianHang(string tendangnhap, string gianhang)
        {
            LichSuGianHang lichsugianhang = new LichSuGianHang();
            TaiKhoan       taikhoan       = context.TaiKhoan.Where(tk => tk.TenDangNhap == tendangnhap).SingleOrDefault();

            lichsugianhang.Id         = Guid.Parse(Guid.NewGuid().ToString().ToUpper());
            lichsugianhang.IdTaiKhoan = taikhoan.Id;
            lichsugianhang.IdGianHang = Guid.Parse(gianhang);
            lichsugianhang.NgayDangKy = DateTime.Now;
            lichsugianhang.TinhTrang  = "Không khoá";
            context.LichSuGianHang.Add(lichsugianhang);
            context.SaveChanges();
            return("Thêm thành công");
        }