Session State ArrayList In Shopping Cart?

I'm creating a shopping cart application and I'm having some issues with implementing a session state for my arraylist.

in my page load i declared

if (Session["Cart"] == null)
{
Session["Cart"] = new ArrayList();
}
else
{
ArrayList cart = (ArrayList)Session["Cart"];
}

to create the session if it doesn't exist yet.
then i have an event handler for a button to add items to the arraylist

protected void onClick_AddBooking(object sender, EventArgs e)
{
int ClassID = Convert.ToInt32(Request.QueryString.Get("Class_Id"));
ArrayList cart1 = new ArrayList();
cart1 = Session["Cart"];
cart1.Add(ClassID);

I'm essentially storing the class_ID then when the student confirms i'll store that to the DB and associate that ID with the Class Details.