ASP.NET

protected void BtnUpdate_Click(object sender, EventArgs e)
{
    dt = (DataTable)Session["Cart"];
    CheckBox chkSelected = new CheckBox();

    foreach (DataGridItem dgItem in DataGrid2.Items)
    {
        chkSelected = (CheckBox)dgItem.FindControl("chkSelection");

        if (chkSelected.Checked == true)
        {


            TextBox TxtQTY = (TextBox)dgItem.Cells[2].FindControl("TxtQTY");
            int Txt2 = Convert.ToInt32(TxtQTY.Text);

            Label lblProduct_Id = (Label)dgItem.Cells[0].FindControl("lblProduct_Id");
            string id2 = lblProduct_Id.Text;

            Label lblDescription = (Label)dgItem.Cells[1].FindControl("lblDescription");
            string desc2 = lblDescription.Text;

            Label lblPrice = (Label)dgItem.Cells[3].FindControl("lblPrice");
            float price2 = Convert.ToSingle(lblPrice.Text);

            Label lblCost = (Label)dgItem.Cells[3].FindControl("lblCost");
            float cost2 = Convert.ToSingle(lblCost.Text);

            float final_cost = price2 * Txt2;

              DataRow myrow = dt.NewRow();
              myrow["REF"] = id2;
              myrow["Description"] = desc2;
              myrow["QTY"] = Txt2;
              myrow["Price"] = price2;
              myrow["Cost"] = final_cost;

              dt.Rows.Add(myrow);
              dt.AcceptChanges();
              Session["data"] = dt;
        }

    }
    DataGrid2.DataSource = dt;
    DataGrid2.DataBind();
}

if (file.Exists)
{
 Response.Clear();
 Response.AddHeader("Content-Disposition", "attachment; filename=" + fileName);
 Response.AddHeader("Content-Length", file.Length.ToString());
 Response.ContentType = "application/octet-stream";
 Response.WriteFile(file.FullName);            
 Response.End();
}