朋友们可以自行测试，我这里都没有问题了，鳖了一上午的问题总算解决了

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 WebApplication1
{
/// <summary>
/// Summary description for WebForm1.
/// </summary>
public class WebForm1 : System.Web.UI.Page
{
protected System.Web.UI.WebControls.TextBox TextBox1;
protected System.Web.UI.WebControls.DropDownList DropDownList1;
protected System.Web.UI.WebControls.Button Button1;

public WebForm1()
{
Page.Init += new System.EventHandler(Page_Init);
}

private void Page_Load(object sender, System.EventArgs e)
{
if(!Page.IsPostBack)
{
//打开某文件(假设WEB。CONFIG在根目录中)
string filename=Server.MapPath("/") + "\web.config";
XmlDocument  xmldoc= new XmlDocument();
xmldoc.Load(filename);

XmlNodeList topM=xmldoc.DocumentElement.ChildNodes;
foreach(XmlElement element in topM)
{
if(element.Name.ToLower()=="appsettings")
{
XmlNodeList _node=element.ChildNodes;
if ( _node.Count >0 )
{
DropDownList1.Items.Clear();
foreach(XmlElement el in _node)
{
DropDownList1.Items.Add(el.Attributes["key"].InnerXml);
}
}
}
}
}
}

private void Page_Init(object sender, EventArgs e)
{
InitializeComponent();
}

#region Web Form Designer generated code
/// <summary>
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
/// </summary>
private void InitializeComponent()
{   
this.Button1.Click += new System.EventHandler(this.Button1_Click);
this.Load += new System.EventHandler(this.Page_Load);

}
#endregion

private void Button1_Click(object sender, System.EventArgs e)
{
string filename=Server.MapPath("/") + "\web.config";
XmlDocument  xmldoc= new XmlDocument();
xmldoc.Load(filename);

XmlNodeList topM=xmldoc.DocumentElement.ChildNodes;
foreach(XmlElement element in topM)
{
if(element.Name.ToLower()=="appsettings")
{
XmlNodeList _node=element.ChildNodes;
if ( _node.Count >0 )
{
foreach(XmlElement el in _node)
{
if(el.Attributes["key"].InnerXml.ToLower()==this.DropDownList1.SelectedItem.Value.ToLower())
{
el.Attributes["value"].Value=this.TextBox1.Text;
}
}
}
}
}
xmldoc.Save(filename);
}
}
}