XDocument xDocument = XDocument.Load("App.Config");
xDocument.Root.Element("appSettings").Elements("add").ToList().Find(x => x.Attribute("key").Value == "ConnectionString").Attribute("value").Value = "connstr"; 
xDocument.Save("App.Config");

解决方案 »

  1.   

    http://www.cnblogs.com/cpcpc/archive/2012/09/10/2679083.html
      

  2.   

    使用ConfigurationManager就可以处理
                     Configuration configuration = null;                 //Configuration对象
                    AppSettingsSection appSection = null;               //AppSection对象 
                    configuration = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
                    //取得AppSetting节
                    appSection = configuration.AppSettings;
                    //赋值并保存
                    appSection.Settings["DataSourceConnection"].Value = txtDataSourceConnection.Text;
                    ...
                     configuration.Save();