//Event payment
        private void btnPayment_Click(object sender,EventArgs e)
        {
            using (ConvenienceShopEntities entity = new ConvenienceShopEntities())
            {
                Bill bill = new Bill();
                bill.DateOfSale = dtpSaleDate.Value;
                SaveBill(bill);
                Bill tmp = new Bill();
                tmp = entity.Bills.SqlQuery("Select * from Bill order by Id DESC").FirstOrDefault();

                int idx = tmp.Id;

                int result = 0;



                for (int i = 0; i < dgvOrderList.Rows.Count - 1; i++)
                {
                    BillInfo        billInfo = new BillInfo();
                    DataGridViewRow row      = dgvOrderList.Rows[i];
                    billInfo.BillID     = idx;
                    billInfo.TypeID     = entity.TypeOfProducts.SqlQuery("Select * from TypeOfProduct where TypeName = N'" + row.Cells[0].Value.ToString() + "'").FirstOrDefault().Id;
                    billInfo.ProductID  = entity.Products.SqlQuery("Select * from Product where ProductName = N'" + row.Cells[1].Value.ToString() + "'").FirstOrDefault().Id;
                    billInfo.Amount     = Convert.ToInt32(row.Cells[3].Value);
                    billInfo.Discount   = Convert.ToInt32(row.Cells[5].Value);
                    billInfo.TotalPrice = Convert.ToInt32(row.Cells[6].Value);
                    result = SaveOrder(billInfo);

                    List <Product> t = new List <Product>();
                    t = entity.Products.SqlQuery("select * from Product where ProductName= N'" + row.Cells[1].Value.ToString() + "' " +
                                                 " and TypeID=" + billInfo.TypeID + " " +
                                                 " and Amount > 0").ToList();
                    foreach (Product ii in t)
                    {
                        if (billInfo.Amount <= ii.Amount)
                        {
                            entity.Database.ExecuteSqlCommand("update Product set Amount=" + (ii.Amount - billInfo.Amount) + " where Id=" + ii.Id);
                            entity.SaveChanges();
                            break;
                        }
                        entity.Database.ExecuteSqlCommand("update Product set Amount= 0 where Id=" + ii.Id);
                        billInfo.Amount -= ii.Amount;
                    }
                }

                if (result == 1)
                {
                    dgvOrderList.Rows.Clear();

                    MessageBox.Show("Pay successfully!","Message",MessageBoxButtons.OK,MessageBoxIcon.Information);
                }
                else
                {
                    MessageBox.Show("Can not be paid!","Error",MessageBoxButtons.OK,MessageBoxIcon.Error);
                }

                FormSale_Load(sender,e);
            }
        }
        //Function save orders onto database
        public int SaveOrder(BillInfo billInfo)
        {
            int result = 0;

            using (ConvenienceShopEntities entity = new ConvenienceShopEntities())
            {
                entity.BillInfoes.Add(billInfo);
                entity.SaveChanges();
                result = 1;
            }
            return(result);
        }