/// <summary>
  /// 微信许可令牌
  /// </summary>
  public class AccessToken
  {
    /// <summary>
    /// 保存已获取到的许可令牌，键为公众号，值为公众号最后一次获取到的令牌
    /// </summary>
    private static ConcurrentDictionary<string, Tuple<AccessToken, DateTime accessTokens = new ConcurrentDictionary<string, Tuple<AccessToken, DateTime();

    /// <summary>
    /// 获取access token的地址
    /// </summary>
    private const string urlForGettingAccessToken = "https://api.weixin.qq.com/cgi-bin/token";
    /// <summary>
    /// 获取access token的http方法
    /// </summary>
    private const string httpMethodForGettingAccessToken = WebRequestMethods.Http.Get;
    /// <summary>
    /// 保存access token的最长时间（单位：秒），超过时间之后，需要重新获取
    /// </summary>
    private const int accessTokenSavingSeconds = 7000;

    /// <summary>
    /// access token
    /// </summary>
    public string access_token { get; set; }
    /// <summary>
    /// 有效时间，单位：秒
    /// </summary>
    public int expires_in { get; set; }

    /// <summary>
    /// 构造函数
    /// </summary>
    /// <param name="_access_token">access token</param>
    /// <param name="_expires_in">有效时间</param>
    internal AccessToken(string _access_token, int _expires_in)
    {
      access_token = _access_token;
      expires_in = _expires_in;
    }

    /// <summary>
    /// 返回AccessToken字符串
    /// </summary>
    /// <returns></returns>
    public override string ToString()
    {
      return string.Format("AccessToken：{0}\r\n有效时间：{1}秒", access_token, expires_in);
    }

    /// <summary>
    /// 从JSON字符串解析AccessToken
    /// </summary>
    /// <param name="json">JSON字符串</param>
    /// <returns>返回AccessToken</returns>
    internal static AccessToken ParseFromJson(string json)
    {
      var at = JsonConvert.DeserializeAnonymousType(json, new { access_token = "", expires_in = 1 });
      return new AccessToken(at.access_token, at.expires_in);
    }

    /// <summary>
    /// 尝试从JSON字符串解析AccessToken
    /// </summary>
    /// <param name="json">JSON字符串</param>
    /// <param name="msg">如果解析成功，返回AccessToken；否则，返回null。</param>
    /// <returns>返回是否解析成功</returns>
    internal static bool TryParseFromJson(string json, out AccessToken token)
    {
      bool success = false;
      token = null;
      try
      {
        token = ParseFromJson(json);
        success = true;
      }
      catch { }
      return success;
    }

    /// <summary>
    /// 得到access token
    /// </summary>
    /// <param name="userName">公众号</param>
    /// <returns>返回access token</returns>
    public static AccessToken Get(string userName)
    {
      Tuple<AccessToken, DateTime> lastToken = accessTokens.ContainsKey(userName) "userName">公众号</param>
    /// <param name="msg">从服务器返回的错误信息。</param>
    /// <returns>返回许可令牌；如果获取失败，返回null。</returns>
    private static AccessToken GetFromWeixinServer(string userName, out ErrorMessage msg)
    {
      AccessToken token = null;
      msg = new ErrorMessage(ErrorMessage.ExceptionCode, "");
      string url = string.Format(urlForGettingAccessToken, WxPayConfig.APPID, WxPayConfig.APPSECRET);
      string result;
      if(!HttpHelper.Request(url, out result, httpMethodForGettingAccessToken, string.Empty))
      {
        msg.errmsg = "从微信服务器获取响应失败。";
        return token;
      }
      if (ErrorMessage.IsErrorMessage(result))
        msg = ErrorMessage.Parse(result);
      else
      {
        try
        {
          token = AccessToken.ParseFromJson(result);
        }
        catch (Exception e)
        {
          msg = new ErrorMessage(e);
        }
      }
      return token;
    }
  }