Запись элемента в XML-файл

private void button_ob_sh_Click(object sender, EventArgs e)
{
 string[] spisok_num_soed = {
         "Connection1", "Connection2", "Connection3", "Connection4",
         "Connection5", "Connection6", "Connection7", "Connection8",  
         "Connection9", "Connection10", "Connection11", "Connection12",
         "Connection13", "Connection14", "Connection15", "Connection16",
         "Connection17", "Connection18", "Connection19", "Connection20" };

 var doc = XDocument.Load("DOS.xml");
 var lastNode = doc.Root.LastNode as XElement;
 var idx = (lastNode == null) ? -1 : Array.BinarySearch(spisok_num_soed, lastNode.Name.LocalName);

  XElement xConnections = new XElement(spisok_num_soed[++idx],
                             new XElement("Element1", comboBox1_el_sh.Text,
                               new XAttribute("attr1_el1", textBox1.Text)),

                             new XElement("Relation", comboBox_soed.Text,
                               new XAttribute("attr1_rel", textBox2.Text)),

                             new XElement("Element2", comboBox2_el_sh.Text,
                               new XAttribute("attr1_el2", textBox3.Text)));

doc.Root.Add(xConnections);

doc.Save("DOS.xml");
}