if(!System.IO.File.Exists("myxml.xml")){
//myxml.xml不存在;
System.Xml.XmlTextWriter writer=new System.Xml.XmlTextWriter(System.IO.File.Open("myxml",FileMode.Create),System.Text.Encoding.GetEncoding("gb2312"));
writer.WriteStartDocument();
writer.Formatting = Formatting.Indented
writer.WriterStartElement("myelement");
....
writer.WriteEndElement();
}