c#来修改XML文档中某个接点的值

XmlDocument doc = new XmlDocument();
doc.load("c:\\a.xml");
XmlNode node = doc.selectSingleNode("root");
node.firstChild.value = "aaaa";
doc.save("c:\\a.xml");

using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using System.Xml ;
namespace DataGridTest
{
/// <summary>
/// XML 的摘要说明。
/// </summary>
public class XML : System.Web.UI.Page
{
protected System.Web.UI.WebControls.Button btadd;
protected System.Web.UI.WebControls.Button btedit;
protected System.Web.UI.WebControls.Button btdelete;
protected System.Web.UI.WebControls.Button btselect;
private XmlDocument xmldoc; <