private void buttonSaveBook_Click(object sender, EventArgs e)
        {
            List <Inventory> listI = InventoryDA.ListInventory();

            if (IsValidInventoryData())
            {
                if (!Validation.IsUniqueISBN(listI, Convert.ToInt32(textBoxISBN.Text)))
                {
                    MessageBox.Show("Duplicate", "DUPLICATE Author ID");
                    textBoxISBN.Clear();
                    textBoxISBN.Focus();
                    return;
                }
                Inventory aProducts = new Inventory();
                aProducts.ISBNProduct          = Convert.ToInt32(textBoxISBN.Text);
                aProducts.ProductTitle         = textBoxBookTitle.Text;
                aProducts.ProductDescription   = comboboxProductDescription.Text;
                aProducts.LastName             = comboBoxAuthorId.Text;
                aProducts.PublisherName        = comboBoxPublisherName.Text;
                aProducts.ProductYearPublished = Convert.ToInt32(textBoxYearPushlished.Text);
                aProducts.ProductQuantity      = Convert.ToInt32(textBoxQuantityOnHand.Text);
                aProducts.ProductPrice         = Convert.ToDouble(textBoxBookPrice.Text);
                InventoryDA.Save(aProducts);
                listB.Add(aProducts);
                buttonListBook.Enabled = true;
                ClearAll();
            }
        }