C#： winform 不能修改app.config

查了很多文章都介紹用下面的方法修改app.config中的值：
Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
config.AppSettings.Settings["DB_UID"].Value = "demo";
config.Save(ConfigurationSaveMode.Full);
事實證明這種做法是不可行的。雖然程式運行時的值已經被修改。但是開啟app.config查看，會看到值仍然沒有被改動。如果重啟程式，就會使用以前的舊值。用這種方法只可以讀取值。不能寫入或者修改值。

正確的做法是把app.config當成一般的xml檔案，進行修改。
XmlDocument xDoc = new XmlDocument();
xDoc.Load(System.Windows.Forms.Application.ExecutablePath + ".config");
XmlNode xNode;
XmlElement xElemUID;
xNode = xDoc.SelectSingleNode("//appSettings");
xElemUID = (XmlElement)xNode.SelectSingleNode("//add[@key='DB_UID']");
xElemUID.SetAttribute("value", Encrypt.Create().encryptData(this.dbUserTXT.Text));
xDoc.Save(System.Windows.Forms.Application.ExecutablePath + ".config");

       private void UpdateConfig(string Xvalue)
        {
            XmlDocument doc = new XmlDocument();
            doc.Load(AppConfig());
            XmlNode node = doc.SelectSingleNode(@"//appSettings");

            XmlElement ele = (XmlElement)node.SelectSingleNode(@"//add[@key='BIRelease']");
            ele.SetAttribute("value", Application.StartupPath);
            doc.Save(AppConfig());

           
        }

        public  string AppConfig()
        {
            int intPos = Application.StartupPath.Trim().IndexOf("bin")-1;

            string strDirectoryPath = System.IO.Path.Combine(Application.StartupPath.Substring(0,intPos), "App.config");

            return strDirectoryPath;
        }

        public  string GetValue(string appKey)
        {
            XmlDocument xDoc = new XmlDocument();
            try
            {
                xDoc.Load(AppConfig());
                XmlNode xNode;
                XmlElement xElem;
                xNode = xDoc.SelectSingleNode("//appSettings");　　　　//補充，需要在你的app.config 檔案中增加一下，<appSetting> </appSetting>
                xElem = (XmlElement)xNode.SelectSingleNode("//add[@key='" + appKey + "']");
                if (xElem != null)
                    return xElem.GetAttribute("value");
                else
                    return "";
            }
            catch (Exception)
            {
                return "";
            }
        }