My solution is here
dsExport.WriteXml(_appath + "\\" + strfilename);
StreamReader streamReader;
streamReader = File.OpenText(_appath + "\\" + strfilename);
string contents = streamReader.ReadToEnd();
streamReader.Close();
StreamWriter streamWriter = File.CreateText(_appath + "\\" + strfilename);
streamWriter.Write(contents.Substring(contents.IndexOf("?>")+2));
streamWriter.Close();
Thanks