C# 如何获取自定义的config中节点的值，并修改节点的值

using System;
using System.Xml;
using System.Configuration;
using System.Reflection;
using System.Web;
using System.IO;

namespace Chain.Common
{

    /// <summary>
    /// Summary description for ReadWriteConfig.
    /// </summary>
    public class DIYConfigHelper
    {
        /// <summary>
        /// 
        /// </summary>
        /// <param name="key">节点名称</param>
        /// <returns></returns>

        /// <summary>
        /// 获取自定义 index.config 文件中的 appsetting 节点值
        /// </summary>
        /// <param name="path">config文件的路径</param>
        /// <param name="key">节点名称</param>
        /// <returns>节点名称的值</returns>
        public static string GetIndexConfigValue(string path, string key)
        {
            string indexConfigPath = path;
            if (string.IsNullOrEmpty(indexConfigPath))
                throw new Exception("请检查应用程序配置文件 appSettings 节点，是否存在 indexConfig 且 value 不为空的配置节！");
            if (!File.Exists(indexConfigPath))
                throw new Exception(string.Format("配置文件不存在：{0}", indexConfigPath));

            ExeConfigurationFileMap ecf = new ExeConfigurationFileMap();
            ecf.ExeConfigFilename = indexConfigPath;
            Configuration config = ConfigurationManager.OpenMappedExeConfiguration(ecf, ConfigurationUserLevel.None);
            return config.AppSettings.Settings[key].Value;
        }

        /// <summary>
        /// 设置自定义 index.config 文件中的 appsetting 节点值
        /// </summary>
        /// <param name="path">config文件的路径</param>
        /// <param name="key">节点名称</param>
        /// <param name="value">需要修改的值</param>
        /// <returns>true：修改成功 false：修改失败</returns>
        public static bool SetIndexConfigValue(string path, string key, string value)
        {
            string indexConfigPath = path;
            if (string.IsNullOrEmpty(indexConfigPath))
                throw new Exception("请检查应用程序配置文件 appSettings 节点，是否存在 indexConfig 且 value 不为空的配置节！");
            if (!File.Exists(indexConfigPath))
                throw new Exception(string.Format("配置文件不存在：{0}", indexConfigPath));

            ExeConfigurationFileMap ecf = new ExeConfigurationFileMap();
            ecf.ExeConfigFilename = indexConfigPath;
            Configuration config = ConfigurationManager.OpenMappedExeConfiguration(ecf, ConfigurationUserLevel.None);
            config.AppSettings.Settings[key].Value = value;
            config.Save();
            return true;
        }
    }
}

string ss = Chain.Common.DIYConfigHelper.GetIndexConfigValue(HttpRuntime.AppDomainAppPath.ToString() + "DIY.config", "14");
bool tt = Chain.Common.DIYConfigHelper.SetIndexConfigValue(HttpRuntime.AppDomainAppPath.ToString() + "DIY.config", "14", "5");

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <appSettings>
     <add key="15" value="663CFB4AF7AE2A91B14587C31B3DE60AF38AED2E63F5040C5D453CBC704162B8ACDD7A7D67A95FA0" /> 
    <add key="14" value="156D7DB054ABBF9B321B1E8982130FDA3420475BC524C4259C55A8CEA4F884DE649FD16284A1053F" />
  </appSettings>
  <connectionStrings />
</configuration>