finally I succeeded :)
if (!File.Exists("Test.xml"))
{
XmlWriterSettings xmlWriterSettings = new XmlWriterSettings();
xmlWriterSettings.Indent = true;
xmlWriterSettings.NewLineOnAttributes = true;
using (XmlWriter xmlWriter = XmlWriter.Create("Test.xml", xmlWriterSettings))
{
xmlWriter.WriteStartDocument();
xmlWriter.WriteStartElement("School");
xmlWriter.WriteStartElement("Student");
xmlWriter.WriteElementString("FirstName", firstName);
xmlWriter.WriteElementString("LastName", lastName);
xmlWriter.WriteEndElement();
xmlWriter.WriteEndElement();
xmlWriter.WriteEndDocument();
xmlWriter.Flush();
xmlWriter.Close();
}
}
else
{
XDocument xDocument = XDocument.Load("Test.xml");
XElement root= xDocument.Element("School");
IEnumerable<XElement> rows = root.Descendants("Student");
XElement firstRow= rows.First();
firstRow.AddBeforeSelf(
new XElement("Student",
new XElement("FirstName", firstName),
new XElement("LastName", lastName)));
xDocument.Save("Test.xml");
}