public static Sach.sach Nhapsach(Sach.listsach a, string ghichu)
        {
            Console.WriteLine(ghichu);
            Sach.sach book;
            Console.Write("Ma sach_ISBN: ");
nhapmasach:
            string isbn = Console.ReadLine();

            if (FindBook.Tramasach(a, isbn) == -1)
            {
                book.Masach_ISBN = isbn;
            }
            else
            {
                Console.WriteLine("Sach da ton tai.");
                Console.Write("Nhap lai ma sach ISBN: ");
                goto nhapmasach;
            }
            Console.Write("Ten sach: ");
            book.Tensach = Console.ReadLine();
            Console.Write("Tac gia: ");
            book.Tacgia = Console.ReadLine();
            Console.Write("Nha xuat ban: ");
            book.Nhaxuatban = Console.ReadLine();
            Console.Write("Gia sach: ");
            book.Giasach = int.Parse(Console.ReadLine());
nhaploaisach:
            Console.WriteLine("(Sach Tieng Viet - 0; Sach Ngoai van - 1)");
            Console.Write("Loai sach: ");
            int k;

            k = int.Parse(Console.ReadLine());
            if (k == 0)
            {
                book.Loaisach = "Sach Tieng Viet";
            }
            else if (k == 1)
            {
                book.Loaisach = "Sach Ngoai van";
            }
            else
            {
                Console.WriteLine("Loai sach khong ton tai moi nhap lai");
                goto nhaploaisach;
            }

            Console.WriteLine(book.Loaisach);
            Console.Write("So luong: ");
            book.Tongsoluong = int.Parse(Console.ReadLine());
            book.Slmuon      = 0;

            book.Slhientai = book.Tongsoluong;
            return(book);
        }
        public static void Xoasach(Sach.listsach a)
        {
            string MS;
            int    vitri;

            Console.WriteLine("XOA THONG TIN SACH");
            Console.Write("Nhap vao Ma so cua sach:");
            MS    = Console.ReadLine();
            vitri = FindBook.Tramasach(a, MS);
            if (vitri == -1)
            {
                Console.WriteLine("Khong co ma sach nay.");
            }
            else
            {
                int n = a.dssach.Length;
                for (int i = vitri; i <= n; i++)
                {
                    a.dssach[i] = a.dssach[i + 1];
                    n--;
                }
            }
        }