Autopostback resetting my list

Question

I want to store items to a list but everytime i press the confirm button to add one item to the list the page will refresh and reset my list. How do i stop this from happening whilst at the same time have an add button to keep adding to the list?

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
    }
    else
    {
        slist = (List<Shopping>)ViewState["Items"];
    }
}

List:

private static List<Shopping> slist;
        public List<Shopping> GetShopping()
        {
            return slist;
        } 

Button Click:

slist = new List<Shopping>(); 
            Shopping s = new Shopping();
            s.Item1 = txtItem1.Text;
            s.Item2 = txtItem2.Text;
            s.Item3 = txtItem3.Text;
            s.Item4 = txtItem3.Text;

            slist.Add(s);
            ViewState["Items"] = slist;
            showShopping();

Method:

showShopping()
{
GridView1.DataSource = GetShopping();
            GridView1.DataBind();
}

Answer 1

Do not store business objects in ViewState. Use the Session

private List<Shopping> ShoppingList
{
   get { 
          var shopping = Session["Shopping"] as List<Shopping>;
          if (shopping == null)
          {
              shopping = new List<Shopping>();
              Session["Shopping"] = shopping;
          }
          return shopping;
       }
   set { Session["Shopping"] = value; }
}

Button handler

var shopping = ShoppingList;
shopping.Add(new Shopping());
...

Answer 2

Stop building/binding the list on your page load without checking Page.IsPostBack

protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
       // bind / load lists and controls here
    }
    else
    {
       //this is a post back, don't reload everything
    }
}

Edit *

Never do this if you ever want to support more then 1 shopper at a time.

private static List<Shopping> slist; 

The users will end up writing over each others lists, then displaying the wrong one constantly. Remove the 'static' modifier.

Answer 3

Been awhile since I have done WebForms, but it looks like you are assigning multiple items, but only one "Shopping" on the button click event then calling:

showShopping() { GridView1.DataSource = GetShopping(); GridView1.DataBind(); }

Double check your GridView1 binding if you want the items to be displayed versus the Shopping object that you are binding to.