C#为配置文件添加结点

private void addnewnode( string nodekey,string nodevalue)
        ...{
            try
            ...{
                System.Xml.XmlDocument xmlDoc = new System.Xml.XmlDocument();
                xmlDoc.Load(sInstallPath+@"web.config");
                System.Xml.XmlNode node;
                XmlNode fanode=xmlDoc.SelectSingleNode("//appSettings");
                node = xmlDoc.SelectSingleNode(string.Format("//appSettings/add[@key='{0}']",nodekey));
                if(node==null)
                ...{

                    XmlElement connode=xmlDoc.CreateElement("add");
                
                    //connode.Name="add";
                    connode.SetAttribute("key",nodekey);
                    connode.SetAttribute("value",nodevalue);
                    //connode.Attributes["key"].Value=nodekey;
                    //connode.Attributes["value"].Value=nodevalue;
                
                    fanode.AppendChild(connode);
                    xmlDoc.Save(sInstallPath+"web.config");

                }
            }
            catch(Exception er)
            ...{
                MessageBox.Show(er.ToString());
            }
        }