//打开某文件(假设web.config在根目录中)
    string filename=Server.MapPath("/") + @"Webapplication1\web.config";
    xmlDocument xmldoc= new XmlDocument();
    xmldoc.Load(filename);
 
    //得到顶层节点列表
    XmlNodeList topM=xmldoc.DocumentElement.ChildNodes;
    foreach(XmlElement element in topM)
    {
     if(element.Name.ToLower()=="appsettings")
     {
 
      //得到该节点的子节点
      XmlNodeList nodelist=element.ChildNodes;
 
      if ( nodelist.Count >0 )
      {
       //DropDownList1.Items.Clear();
 
       foreach(XmlElement el in nodelist)//读元素值
       {
        //DropDownList1.Items.Add(el.Attributes["key"].InnerXml);
        //this.TextBox2.Text=el.Attributes["key"].InnerText;
        this.TextBox2.Text=el.Attributes["key"].Value;
        this.Label1.Text=el.Attributes["value"].Value;
 
 
            //同样在这里可以修改元素值,在后面save。
         //  el.Attributes["value"].Value=this.TextBox2.Text;
       }
 
      }
     }
    }

 if(element.Name.ToLower()=="appsettings")
    {
 
     XmlElement elem =xmldoc.CreateElement("add");
     
     element.AppendChild(elem);
     elem.InnerText="ltp";
 
     xmldoc.Save(filename);
       
    }

<appSettings>
    <add key="密码" value="admin" />
    <add>ltp</add>
  </appSettings>

 if(element.Name.ToLower()=="appsettings")
    {
 
     XmlElement elem =xmldoc.CreateElement("add");
     element.AppendChild(elem);
 
     XmlAttribute xa=xmldoc.CreateAttribute("key");
     xa.Value="ltp";
 
     XmlAttribute xa2=xmldoc.CreateAttribute("value");
     xa2.Value="first";
 
     elem.SetAttributeNode(xa);
     elem.SetAttributeNode(xa2);
 
     xmldoc.Save(filename);
       
    }

  <appSettings>
    <add key="密码" value="admin" />
    <add key="ltp" value="first" />
  </appSettings>
 
//添加空元素：
   XmlNode node=doc.CreateElement(groupname);
     node.InnerText="";
     doc.LastChild.AppendChild(node);
 
     doc.Save(xmlfile);

string itemname=this.listBox1.SelectedItem.ToString();
   
   this.listBox1.Items.Remove(this.listBox1.SelectedItem);
 
   //begin del xmlfile
   XmlDocument doc=new XmlDocument();
   doc.Load(xmlfile);
   
   XmlNodeList topM=doc.DocumentElement.ChildNodes;
   foreach(XmlElement element in topM)
   {
    if(element.Name==this.comboBox1.Text)
    {
 
     //得到该节点的子节点
     XmlNodeList nodelist=element.ChildNodes;      
 
     foreach(XmlElement el in nodelist)//读元素值
     {       
      if(el.Attributes["key"].Value==itemname)
      {
       element.RemoveChild(el);
      }
 
     }//循环元素
      
    }//得到组
 
   }//循环组
   doc.Save(xmlfile);  //一定要保存一下，否则不起作用
 
//筛选数据
PRivate void Reader_Xml(string pathFlie)
{
   XmlDocument Xmldoc=new XmlDocument();
   Xmldoc.Load(pathFlie);
   XmlNodeList Record1=Xmldoc.DocumentElement.SelectNodes(Code[@id='1'])
   int f=0;
   foreach(XmlNode xnode in Record1)
    {
 
    }
} /**//*读取xml数据   两种xml方式*/
<aaa>
     <bb>something</bb>
     <cc>something</cc>
</aaa>
 
<aaa>
    <add key="123" value="321"/>
</aaa>
/**//*第一种方法*/
DS.ReadXml("your xmlfile name");
Container.DataItem("bb");
Container.DataItem("cc");
DS.ReadXmlSchema("your xmlfile name");
 
/**//*第二种方法*/
<aaa>
    <add key="123" value="321"/>
</aaa>

using System.XML;
XmlDataDocument xmlDoc = new System.Xml.XmlDataDocument();
xmlDoc.Load(@"c:\Config.xml");
XmlElement elem = xmlDoc.GetElementById("add");
string str = elem.Attributes["value"].Value
 
 
/**//*第三种方法:  SelectSingleNode  读取两种格式的xml *---/
--------------------------------------------------------------------
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <appSettings>
       <ConnectionString>Data Source=yf; user id=ctm_dbo;passWord=123</ConnectionString>             
  </appSettings>
</configuration>
--------------------------------------------------------------------------
XmlDocument doc = new XmlDocument();
doc.Load(strXmlName);
 
    XmlNode node=doc.SelectSingleNode("/configuration/appSettings/ConnectionString");
    if(node!=null)
    {
     string k1=node.Value;    //null
     string k2=node.InnerText;//Data Source=yf; user id=ctm_dbo;password=123
     string k3=node.InnerXml;//Data Source=yf; user id=ctm_dbo;password=123
     node=null;
    }
 
********************************************************************
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <appSettings>
       <add key="ConnectionString" value="Data Source=yf; user id=ctm_dbo;password=123" />             
  </appSettings>
</configuration>
**--------------------------------------------------------------------**
     XmlNode node=doc.SelectSingleNode("/configuration/appSettings/add");
    if(node!=null)
    {
     string k=node.Attributes["key"].Value;
     string v=node.Attributes["value"].Value;
     node=null;
    }
*--------------------------------------------------------------------*
    XmlNode node=doc.SelectSingleNode("/configuration/appSettings/add");
    if(node!=null)
    {
     XmlNodeReader nr=new XmlNodeReader(node);
     nr.MoveToContent();
    //检查当前节点是否是内容节点。如果此节点不是内容节点，则读取器向前跳至下一个内容节点或文件结尾。
     nr.MoveToAttribute("value");
     string s=nr.Value;
     node=null;
    }