private void codeActivity1_ExecuteCode(object sender, EventArgs e)
    {
    using (SPWeb web = SPContext.Current.Web)
    {
    SPList list = web.Lists["Manager"];
    foreach (SPListItem item in list.Items)
    {
        if (item["Address"] == null)
        {
            item["Address"] = "PLEASE SET THE ADDRESS!";

            item.Update();
        }
      }
     }
    }