今天看到一招用DataSet修改WebConfig，真是简洁明了，一同分享

     public   void  write()

         {

             string  newValue =   this .TextBox1.Text;

            DataSet ds  =   new  DataSet();

            ds.ReadXml(Server.MapPath( "" ) + " /web.config " );

            ds.Tables[ 1 ].Rows[ 0 ][ 1 ]  =  key1;

            ds.AcceptChanges();

            ds.WriteXml(Server.MapPath( "" ) + " /web.config " );

            ds.Dispose();

            

        }