problem with updating the quantity in shopping cart

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C#

ASP.NET

i am trying to make a shopping cart. i displayed simply product id product name unit price quantity using this sql query

string query = "select Products.ProductID,Products.ProductName,Products.UnitCost,ShoppingCart.Quantity from Products RIGHT JOIN ShoppingCart on Products.ProductID=ShoppingCart.ProductID where CustomerID='"+User.Identity.Name+"'"; 

now i have a update button in a cart..

when i update the quantity .. it simple update the quantity in shopping cart table and show the total amount in a label which i put at the footer template of a quntity in gridview.. here is the code. but the cart is not even updating..
there is the error index is out of range exception

can anyone figure out the problem??
here is the code

protected void btnupdatecart_Click(object sender, EventArgs e)
    {
        try
        {
            for (int i = 0; i <= GridView1.Rows.Count - 1; i++)
            {
                GridViewRow row = GridView1.Rows[i];
                if (row.RowType == DataControlRowType.DataRow)
                {
                    ShoppingCartItem item = new ShoppingCartItem();
                    item.CustomerID = User.Identity.Name;
                    //item.ProductID = (int)GridView1.DataKeys[i].Value;
                    item.ProductID = int.Parse(((Label)(row.FindControl("lblproid"))).Text);
                    item.Quantity = int.Parse(((TextBox)(row.FindControl("txtdmainqty"))).Text);
                    if (item.Quantity <= 0)
                    {
                        throw new Exception("Invalid Quantity");
                    }
                    ShoppingCart.UpdateItem(item);
                }
            }
           // GridView1.DataBind();
            bind();
            btnplaceorder.Enabled = true;
            //lblMsg.Text = "";
        }
        catch (Exception ex)
        {
            btnplaceorder.Enabled = false;
            //lblMsg.Text = ex.Message;
        }
    }

    private decimal decTotal;
    protected void _rowdatabound(object sender, GridViewRowEventArgs e)
    {
         if (e.Row.RowType == DataControlRowType.DataRow)
        {
            //int prodid = int.Parse(e.Row.Cells[0].Text);
             int prodid=int.Parse(((Label)(e.Row.FindControl("lblproid"))).Text);
            int qty = int.Parse(((TextBox)(e.Row.FindControl("txtmainqty"))).Text);
            Product p = Product.GetProduct(prodid);
            decTotal = decTotal + (qty * p.UnitCost);
        }
        if (e.Row.RowType == DataControlRowType.Footer)
        {
            ((Label)(e.Row.FindControl("lbltotal"))).Text = "Total :" + decTotal.ToString("C");
        } 

    }
}