高分求解，怎样处理保存成XML文件

Question

public DataSet ds; //写入文件
private void writeXml()
{
try
{
FileStream fs = new FileStream(filename,FileMode.OpenOrCreate);
XmlTextWriter xtw = new XmlTextWriter(fs, Encoding.Unicode);
ds.WriteXml(xtw, XmlWriteMode.IgnoreSchema);
//ds.WriteXml(xtw, XmlWriteMode.WriteSchema);
xtw.Close();
fs.Close();
}
catch(Exception ex)
{
MessageBox.Show (ex.ToString(),"错误提示",MessageBoxButtons.OK ,MessageBoxIcon.Asterisk,MessageBoxDefaultButton.Button1);
}
}