读写xml文件结点值

public static  void UpdateConfig(string item,string itemvalue)
        {
            try
            {
                //将连接字符串写入Web.config
                System.IO.FileInfo fileInfo=new FileInfo(AppDomain.CurrentDomain.BaseDirectory+"DataBase.xml");

                if(!fileInfo.Exists)
                    MessageBox.Show("can't find the app.config");

                XmlDocument doc=new XmlDocument();
                doc.Load(fileInfo.FullName);
                    //doc.Load(AppDomain.CurrentDomain.SetupInformation.ConfigurationFile);
                
                bool foundIt=false;

                string connString=itemvalue;

                string enCS=connString;//SecurityHelper.EncryptDBConnectionString(connString);

                XmlNode no=doc.SelectSingleNode("//appSettings/add[@key='"+item+"']");
                if(no!=null)
                {
                    no.Attributes.GetNamedItem("value").Value=enCS;
                    foundIt=true;
                }

                if(!foundIt)
                    MessageBox.Show("can't find the connString setting ");
                doc.Save(fileInfo.FullName);
            }
            catch(Exception ex)
            {
                StreamWriter w=new StreamWriter(AppDomain.CurrentDomain.BaseDirectory+"log.txt",true);
                w.WriteLine("===in updata connstring=tjtj=====");
                w.WriteLine(ex.ToString());
                w.WriteLine(ex.StackTrace);
                w.Close();
            }
        }
        public static  string  LoadConfig(string item)
        {
            string reval="";
            try
            {
                //将连接字符串写入Web.config
                System.IO.FileInfo fileInfo=new FileInfo(AppDomain.CurrentDomain.BaseDirectory+"DataBase.xml");

                if(!fileInfo.Exists)
                    MessageBox.Show("can't find the app.config");

                XmlDocument doc=new XmlDocument();
                doc.Load(fileInfo.FullName);
                //doc.Load(AppDomain.CurrentDomain.SetupInformation.ConfigurationFile);
                
                bool foundIt=false;
                
                

                

                XmlNode no=doc.SelectSingleNode("//appSettings/add[@key='"+item+"']");
                if(no!=null)
                {
                    reval=no.Attributes.GetNamedItem("value").Value;
                    
                    foundIt=true;
                }

                if(!foundIt)
                    MessageBox.Show("can't find the connString setting ");                
            }
            catch(Exception ex)
            {
                StreamWriter w=new StreamWriter(AppDomain.CurrentDomain.BaseDirectory+"log.txt",true);
                w.WriteLine("===in updata connstring=tjtj=====");
                w.WriteLine(ex.ToString());
                w.WriteLine(ex.StackTrace);
                w.Close();                
            }
            return reval;
        }