Many times we need to export our data in xml format, but writing the xml data to a predefined xml file already included  can cause concurrency problem  for multiple users accessing the same file, also it is not a good idea to generate xml file with multiple names in the  server as it will overload server memory,
So the best idea is to generate xml file on the fly and download it to the requested browser

public void  ExportXML(string BookId)
       {
           

           XmlDocument doc = new XmlDocument();

           // XML declaration
           XmlNode declaration = doc.CreateNode(XmlNodeType.XmlDeclaration, null, null);
           doc.AppendChild(declaration);

           // Root element: Catalog
           XmlElement root = doc.CreateElement("Catalog");
           doc.AppendChild(root);

           // Sub-element: srsapiversion of root
           XmlElement book = doc.CreateElement("book");
           root.AppendChild(book);


           // Sub-element: srsapiversion of root
           XmlElement bookname = doc.CreateElement("bookname");
           book.InnerText = "2 States";
           root.AppendChild(bookname);

           // Sub-element: id of root
           XmlElement id = doc.CreateElement("id");
           id.InnerText = "70-515";
           root.AppendChild(id);

           // Sub-element: author of root
           XmlElement author = doc.CreateElement("author");
           author.InnerText = "Chetan Bhagat";
           //Attribute age of author
           XmlAttribute age = doc.CreateAttribute("age");
           age.Value = "43";
           author.Attributes.Append(age);
           root.AppendChild(author);



           System.IO.MemoryStream stream = new System.IO.MemoryStream();
           XmlTextWriter writer = new XmlTextWriter(stream, System.Text.Encoding.UTF8);

             doc.WriteTo(writer);
               writer.Flush();
               Response.Clear();
               byte[] byteArray = stream.ToArray();
               Response.AppendHeader("Content-Disposition", "filename=Books.xml");
               Response.AppendHeader("Content-Length", byteArray.Length.ToString());
               Response.ContentType = "application/octet-stream";
               Response.BinaryWrite(byteArray);
               writer.Close();

               }