C-Sharp: XML Anfänger benötigt Hilfestellung

//XML functions
        //Export
        private void ExportXML()
        {
            try
            {
                List<string> todoList = new List<string>();
                List<string> erledigtList = new List<string>();
                XmlSerializer serializer = new XmlSerializer(typeof(List<string>));
                TextWriter tw = new StreamWriter(Application.StartupPath + "\\savefile.xml");  

                for (int i = 0; i <= lbTodo.Items.Count - 1; i++)
                {
                    todoList.Add(lbTodo.Items[i].ToString());
                }

                for (int n = 0; n <= lbErledigt.Items.Count - 1; n++)
                {
                    erledigtList.Add(lbErledigt.Items[n].ToString());
                }
                serializer.Serialize(tw, todoList);
                serializer.Serialize(tw, erledigtList);
                tw.Close();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message.ToString());
            }
        }

        //Import
        private void ImportXML()
        {
            try
            {
                XmlDocument xmldoc = new XmlDocument();
                xmldoc.Load(Application.StartupPath + "\\savefile.xml");  
                XmlElement root = xmldoc.DocumentElement;
                XmlNodeList lst = root.GetElementsByTagName("string");  

                foreach (XmlNode n in lst)
                {
                    lbTodo.Items.Add(n.InnerText);
                    lbErledigt.Items.Add(n.InnerText);
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message.ToString());
            }
        }

if (chkTodo.Checked == true)
{
     lbTodo.Items.Add(tbEintrag.Text);
     ExportXML();
}