http://www.cnblogs.com/chenzz/archive/2008/03/28/1127325.html
public void SaveConfig(string key, string value)
{
try
{
string fileName = "Web.Config";
XmlDocument document = new XmlDocument();
document.Load(fileName);
((XmlElement)document.SelectSingleNode(@"/AppSetting/add[@key='" + key + "']")).SetAttribute("Value", value);
document.Save(fileName);
}
catch (Exception ex)
{
throw new Exception(ex.Message);
}
}