Selecting an item in a GridView and adding to the List

I have written the piece of code below to get a productID from a gridView when the user clicks on the select link.

Convert.ToInt32(GridView1.SelectedDataKey.Values["productID"])))

However, if a user clicks on this more than one click the newer value replaces the previous. Is there a way to keep adding to the cart list when the user clicks on a new item?

Hope that makes sense

Thanks

Edit:

Here's my code for the Shopping Page:

public partial class _Default : System.Web.UI.Page
{
    List<BasketClass> cart = new List<BasketClass>();

    protected void GridView1_SelectedIndexChanged1(object sender, EventArgs e)
    {
        cart.Add(new BasketClass(Convert.ToInt32(GridView1.SelectedDataKey.Values["BookID"])));

        Session.Add("CartSess", cart);
        Response.Redirect("Basket.aspx");
    }
}

I dont know if the location of creating the list is important? Wasn't sure if it was placed in the click event if it would keep creating a new instance?

Then fo开发者_C百科r the Basket Page I have:

protected void Page_Init(object sender, EventArgs e)
{
    List<BasketClass> cart = (List<BasketClass>)Session["CartSess"];

    foreach (BasketClass BookID in cart)
    {     
        GridView1.DataSource = cart;
        GridView1.DataBind();
        AccessDataSource1.SelectCommand = "SELECT [BookID], [book_title] FROM [tblBook] ";                     
    }
}

You should not create BasketCart list object in the top of page, even if you do that then there is no very much significant difference, i think you cna do this:

protected void Page_Init(object sender, EventArgs e)
{    
    if(Session["CartSess"]!=null)
    {    
       foreach (BasketClass BookID in (List<BasketClass>)Session["CartSess"])
       {     
         GridView1.DataSource = cart;
         GridView1.DataBind();
        AccessDataSource1.SelectCommand = "SELECT [BookID], [book_title] FROM [tblBook] ";                     
       }
    }
}

and your GridView even should be:

protected void GridView1_SelectedIndexChanged1(object sender, EventArgs e)
{
   List<BasketClass> cart;
   if(Session["CartSess"]!=null)
   { 
       cart = (List<BasketClass>)Session["CartSess"]
   }
   else
     cart = new List<BasketClass>();

   cart.Add(new BasketClass(Convert.ToInt32(GridView1.SelectedDataKey.Values["BookID"])));

   Session.Add("CartSess", cart);
   Response.Redirect("Basket.aspx");
}

It is not this line that is causing your issue, it is where you are storing it. Are you passing this value into a collection?

use a List<int> persisted in ViewState?

public List<int> Selected
{
    set { ViewState["_selected"] = value; }
    get
    {
        if(ViewState["_selected"] == null)
            return new List<int>();

        return (List<int>)ViewState["_selected"];
    }    
}