用c#写XML文件问题

请高手帮我用c#写一个像下面格式的XML文件：

<?xml version="1.0" encoding="GBK" ?>
- <result>

  <authentication>true </authentication>

  <userrole>student </userrole>

  </result>

---

XmlWriter xw=XmlWriter("x.xml");
xw.WriteStartElement("result")
xw.WriteElementString("authentication","true");
xw.WriteElementString("userrole","student");
xw.WriteEndElement();

xw.Flush();
xw.Close();

---

C# code:

private void button2_Click(object sender, EventArgs e)
{
XmlWriterSettings settings = new XmlWriterSettings();
settings.Indent = true;
settings.Encoding = Encoding.GetEncoding("gbk");//只能得到 gb2312
XmlWriter writer = XmlWriter.Create("test.xml", settings);
writer.WriteStartDocument();
writer.WriteStartElement("result");

writer.WriteStartElement("authentication");
writer.WriteString("true");
writer.WriteEndElement();

writer.WriteStartElement("userrole");
writer.WriteString(&q