this is the class i'm working with

  public partial class MainShoppingPage : System.Web.UI.Page
    {
        List<CartObject> cart = new List<CartObject>();
        List<Product> allProducts = new List<Product>();
        List<Product> bookList = new List<Product>();
        List<Product> dvdList = new List<Product>();
        protected void Page_Load(object sender, EventArgs e)
        {
            BuildProducts bp = new BuildProducts();
            allProducts = (List<Product>)Session["AllProducts"];

            if (!IsPostBack)
            {
                foreach (var i in allProducts)
                {
                    if (i.ProductType == "Book")
                    {
                        BooksDropDown.Items.Add(i.Title);
                        BooksDropDown.SelectedIndex = 0;
                        bookList.Add(i);
                        int selectedBook = BooksDropDown.SelectedIndex;
                        BookPriceValueLabel.Text = bookList[selectedBook].Price.ToString("c");
                    }
                    else
                    {
                        DvdDropDown.Items.Add(i.Title);
                        DvdDropDown.SelectedIndex = 0;
                        dvdList.Add(i);
                        int selectedDvd = DvdDropDown.SelectedIndex;
                        DVDPriceValueLabel.Text = dvdList[selectedDvd].Price.ToString("c");
                    }
                }
            }
        }

        //Exit Button
        protected void ExitButton_Click(object sender, EventArgs e)
        {
            Response.Redirect("ExitScreen.aspx");
        }

	// this is the button that i'm having trouble with
        protected void AddButton_Click(object sender, EventArgs e)
        {
            string bookQuantity = BookQuantityTB.Text;
            string dvdQuantity = DvdQuantityTB.Text;
            Product selectedBook = allProducts[BooksDropDown.SelectedIndex];
            Product selectedDvd = allProducts[DvdDropDown.SelectedIndex + 5];

            MessageLabel.Text = "";

            if(Session["CartObjs"] == null)
            {
                if(bookQuantity != "" && dvdQuantity == "")
                {
                    CartObject book = new CartObject(int.Parse(bookQuantity), selectedBook);
                    cart.Add(book);
                    HttpContext.Current.Session["CartObjs"] = cart;
                }
                else if(dvdQuantity != "" && bookQuantity == "")
                {
                    CartObject dvd = new CartObject(int.Parse(dvdQuantity), selectedDvd);
                    cart.Add(dvd);
                    HttpContext.Current.Session["CartObjs"] = cart;
                }
                else if( dvdQuantity  != "" && bookQuantity != "")
                {
                    CartObject book = new CartObject(int.Parse(bookQuantity), selectedBook);
                    CartObject dvd = new CartObject(int.Parse(dvdQuantity), selectedDvd);
                    cart.Add(book);
                    cart.Add(dvd);
                    HttpContext.Current.Session["CartObjs"] = cart;
                }
                else
                {
                    MessageLabel.Text = "One of the quantities must be more than 0. Otherwise Please Click Exit.";
                }
            }

        }