public class ConfigureAppConfig
    {
        //静态构造,不能实例化
        static ConfigureAppConfig() { }

        /// <summary>
        /// 获取AppSettings配置节中的Key值
        /// </summary>
        /// <param name="keyName">Key's name</param>
        /// <returns>Key's value</returns>
        public static string GetAppSettingsKeyValue(string keyName)
        {
            return ConfigurationManager.AppSettings.Get(keyName);
        }

        /// <summary>
        /// 获取ConnectionStrings配置节中的值
        /// </summary>
        /// <returns></returns>
        public static string GetConnectionStringsElementValue()
        {
            ConnectionStringSettings settings = System.Configuration.ConfigurationManager.ConnectionStrings["connectionString"];
            return settings.ConnectionString;
        }

        /// <summary>
        /// 保存节点中ConnectionStrings的子节点配置项的值
        /// </summary>
        /// <param name="elementValue"></param>
        public static void ConnectionStringsSave(string ConnectionStringsName, string elementValue)
        {
            System.Configuration.Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
            config.ConnectionStrings.ConnectionStrings["connectionString"].ConnectionString = elementValue;
            config.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection("connectionStrings");
        }

        /// <summary>
        /// 判断appSettings中是否有此项
        /// </summary>
        private static bool AppSettingsKeyExists(string strKey, Configuration config)
        {
            foreach (string str in config.AppSettings.Settings.AllKeys)
            {
                if (str == strKey)
                {
                    return true;
                }
            }
            return false;
        }

        /// <summary>
        /// 保存appSettings中某key的value值
        /// </summary>
        /// <param name="strKey">key's name</param>
        /// <param name="newValue">value</param>
        public static void AppSettingsSave(string strKey, string newValue)
        {
            System.Configuration.Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
            if (AppSettingsKeyExists(strKey, config))
            {
                config.AppSettings.Settings[strKey].Value = newValue;
                config.Save(ConfigurationSaveMode.Modified);
                ConfigurationManager.RefreshSection("appSettings");
            }
        }
    }

如果你的程序是对其它程序的配置文件进行操作,代码如下：

            ExeConfigurationFileMap filemap = new ExeConfigurationFileMap();
            filemap.ExeConfigFilename = filePath;//配置文件路径
            config = ConfigurationManager.OpenMappedExeConfiguration(filemap, ConfigurationUserLevel.None);

            if (AppSettingsKeyExists("Refresh", config))
            {
                config.AppSettings.Settings["Refresh"].Value = M_TimeRead.ToString();
            }

            if (AppSettingsKeyExists("MachineNo", config))
            {
                config.AppSettings.Settings["MachineNo"].Value = M_MachineNo;

            }
            config.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection("appSettings");

            config.ConnectionStrings.ConnectionStrings["connectionString"].ConnectionString = M_ConnectionString;
            config.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection("connectionStrings");

数据库字符串加密

ExeConfigurationFileMap filemap = new ExeConfigurationFileMap();
            filemap.ExeConfigFilename = Application.ExecutablePath + ".Config"; //filePath;
            config = ConfigurationManager.OpenMappedExeConfiguration(filemap, ConfigurationUserLevel.None);
            //指定我所要的节点
            ConfigurationSection section = config.ConnectionStrings;

            if ((section.SectionInformation.IsProtected == false) && (section.ElementInformation.IsLocked == false))
            {
                //制定节点加密
                section.SectionInformation.ProtectSection(protect);
                //即使没有修改也保存设置
                section.SectionInformation.ForceSave = true;
                //配置文件内容保存到xml
                config.Save(ConfigurationSaveMode.Full);
            }

http://www.cnblogs.com/wangsu/archive/2008/02/25/1081226.html

读取web.config的最好用
Configuration config = WebConfigurationManager.OpenWebConfiguration("~");
其他一样。