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);
}
}
}