How to solve the access_token expiration problem in .Net WeChat development

/// <summary>
///Access_token 的摘要说明
/// </summary>
public class Access_token
{
 public Access_token()
 {
 //
 //TODO: 在此处添加构造函数逻辑
 //
 }
 string _access_token;
 string _expires_in;

 /// <summary>
 /// 获取到的凭证 
 /// </summary>
 public string access_token
 {
  get { return _access_token; }
  set { _access_token = value; }
 }

 /// <summary>
 /// 凭证有效时间，单位：秒
 /// </summary>
 public string expires_in
 {
  get { return _expires_in; }
  set { _expires_in = value; }
 }
}

<?xml version="1.0" encoding="utf-8"?>
<xml>
 <Access_Token>初始值可以随便写</Access_Token>
 <Access_YouXRQ>1980/12/12 16:06:38</Access_YouXRQ>
</xml>

public static Access_token GetAccess_token()
 {
  string appid = 你的appid ;
  string secret = 你的secret;
  string strUrl = "https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid=" + appid + "&secret=" + secret;
  Access_token mode = new Access_token();

  HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create(strUrl);

  req.Method = "GET";
  using (WebResponse wr = req.GetResponse())
  {
   HttpWebResponse myResponse = (HttpWebResponse)req.GetResponse();

   StreamReader reader = new StreamReader(myResponse.GetResponseStream(), Encoding.UTF8);

   string content = reader.ReadToEnd();
   //Response.Write(content);
   //在这里对Access_token 赋值
   Access_token token = new Access_token();
   token = JsonHelper.ParseFromJson<Access_token>(content);
   mode.access_token = token.access_token;
   mode.expires_in = token.expires_in;
  }
  return mode;
 }

using System;
using System.IO;
using System.Text;
using System.Runtime.Serialization.Json;
 

public class JsonHelper
{
 /// <summary>
 /// 生成Json格式
 /// </summary>
 /// <typeparam name="T"></typeparam>
 /// <param name="obj"></param>
 /// <returns></returns>
 public static string GetJson<T>(T obj)
 {
  DataContractJsonSerializer json = new DataContractJsonSerializer(obj.GetType());
  using (MemoryStream stream = new MemoryStream())
  {
   json.WriteObject(stream, obj);
   string szJson = Encoding.UTF8.GetString(stream.ToArray()); return szJson;
  }
 }
 /// <summary>
 /// 获取Json的Model
 /// </summary>
 /// <typeparam name="T"></typeparam>
 /// <param name="szJson"></param>
 /// <returns></returns>
 public static T ParseFromJson<T>(string szJson)
 {
  T obj = Activator.CreateInstance<T>();
  using (MemoryStream ms = new MemoryStream(Encoding.UTF8.GetBytes(szJson)))
  {
   DataContractJsonSerializer serializer = new DataContractJsonSerializer(obj.GetType());
   return (T)serializer.ReadObject(ms);
  }
 }
}

/// <summary>
 /// 根据当前日期 判断Access_Token 是否超期 如果超期返回新的Access_Token 否则返回之前的Access_Token
 /// </summary>
 /// <param name="datetime"></param>
 /// <returns></returns>
 public static string IsExistAccess_Token()
 {

  string Token = string.Empty;
  DateTime YouXRQ;
  // 读取XML文件中的数据，并显示出来 ，注意文件路径
  string filepath = Server.MapPath("XMLFile.xml");

  StreamReader str = new StreamReader(filepath, System.Text.Encoding.UTF8);
  XmlDocument xml = new XmlDocument();
  xml.Load(str);
  str.Close();
  str.Dispose();
  Token = xml.SelectSingleNode("xml").SelectSingleNode("Access_Token").InnerText;
  YouXRQ = Convert.ToDateTime(xml.SelectSingleNode("xml").SelectSingleNode("Access_YouXRQ").InnerText);

  if (DateTime.Now > YouXRQ)
  {
   DateTime _youxrq = DateTime.Now;
   Access_token mode = GetAccess_token();
   xml.SelectSingleNode("xml").SelectSingleNode("Access_Token").InnerText = mode.access_token;
   _youxrq = _youxrq.AddSeconds(int.Parse(mode.expires_in));
   xml.SelectSingleNode("xml").SelectSingleNode("Access_YouXRQ").InnerText = _youxrq.ToString();
   xml.Save(filepath);
   Token = mode.access_token;
  }
  return Token;
 }