ASP.NET รันไม่ได้ครับ รบกวนผู้รู้ช่วยตรวจสอบให้ทีว่าผิดตรงไหน

        DataSet ds;
        protected void Page_Load(object sender, EventArgs e)
        {

            ds = (DataSet)Session["ds"];
            if (!Page.IsPostBack)
            {
                string sql;
                if (!ds.Tables.Contains("cart"))
                {
                    sql = "select* from cart where cartid is null";
                    OleDbDataAdapter da = new OleDbDataAdapter(sql, cn());
                    da.Fill(ds, "cart");
                }
                DataRow[] dr = ds.Tables["cart"].Select("pid='" + Request.QueryString["pid"].Trim() + "'");
                if (dr.Length != 0)
                {
                    dr[0]["qty"] = Convert.ToInt16(dr[0]["qty"]) + 1;//ถ้า๙้ำให้เพิ่มจำนวนเป็น 1 
                    dr[0]["sumprice"] = Convert.ToInt16(dr[0]["qty"]) * Convert.ToInt16(dr[0]["unprice"]); 
                }
                else
                {
                    DataRow drnew = ds.Tables["cart"].NewRow();
                    drnew["pid"] = Request.QueryString["pid"].Trim();
                    DataRow[] dr1 = ds.Tables["product"].Select("p_id='" + Request.QueryString["pid"].Trim() + "'");
                    drnew["pname"] = dr1[0]["p_name"];
                    drnew["qty"] = 1;
                    drnew["pimage"] = dr1[0]["p_picture"];
                    drnew["unitprice"] = dr1[0]["p_price"];
                    drnew["sumprice"] = Convert.ToInt16(dr1[0]["p_price"]);
                    drnew["cartid"] = Session.SessionID;
                    ds.Tables["cart"].Rows.Add(drnew); 
                }
                ds.AcceptChanges(); 
                if (ds.Tables["cart"].Rows.Count == 0)
                {
                    GridView1.Visible = false;
                }
                els 
                {
                    GridView1.Visible = true;
                    GridView1.DataSource = ds.Tables["cart"];
                    DataBind();
                }

            }
        }