/// summary>
  /// 微信許可令牌
  /// /summary>
  public class AccessToken
  {
    /// summary>
    /// 保存已獲取到的許可令牌，鍵為公眾號，值為公眾號最后一次獲取到的令牌
    /// /summary>
    private static ConcurrentDictionarystring, TupleAccessToken, DateTime>> accessTokens = new ConcurrentDictionarystring, TupleAccessToken, DateTime>>();

    /// summary>
    /// 獲取access token的地址
    /// /summary>
    private const string urlForGettingAccessToken = "https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credentialappid={0}secret={1}";
    /// summary>
    /// 獲取access token的http方法
    /// /summary>
    private const string httpMethodForGettingAccessToken = WebRequestMethods.Http.Get;
    /// summary>
    /// 保存access token的最長時間（單位：秒），超過時間之后，需要重新獲取
    /// /summary>
    private const int accessTokenSavingSeconds = 7000;

    /// summary>
    /// access token
    /// /summary>
    public string access_token { get; set; }
    /// summary>
    /// 有效時間，單位：秒
    /// /summary>
    public int expires_in { get; set; }

    /// summary>
    /// 構造函數
    /// /summary>
    /// param name="_access_token">access token/param>
    /// param name="_expires_in">有效時間/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有效時間：{1}秒", access_token, expires_in);
    }

    /// summary>
    /// 從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>
    /// 嘗試從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；否則，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">公眾號/param>
    /// returns>返回access token/returns>
    public static AccessToken Get(string userName)
    {
      TupleAccessToken, DateTime> lastToken = accessTokens.ContainsKey(userName) ? accessTokens[userName] : null;
      AccessToken token = lastToken == null ? null : lastToken.Item1;
      DateTime refreshTime = lastToken == null ? DateTime.MinValue : lastToken.Item2;
      double ms = (DateTime.Now - refreshTime).TotalSeconds;
      if (ms > accessTokenSavingSeconds || token == null)
      {
        //嘗試從微信服務器獲取2次
        ErrorMessage msg;
        AccessToken newToken = GetFromWeixinServer(userName, out msg);
        if (newToken == null)
          newToken = GetFromWeixinServer(userName, out msg);
        if (newToken != null)
        {
          lastToken = new TupleAccessToken, DateTime>(newToken, DateTime.Now);
          accessTokens[userName] = lastToken;
          token = newToken;
        }
      }
      return token;
    }

    /// summary>
    /// 從微信服務器獲取access token
    /// /summary>
    /// param name="userName">公眾號/param>
    /// param name="msg">從服務器返回的錯誤信息。/param>
    /// returns>返回許可令牌；如果獲取失敗，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 = "從微信服務器獲取響應失敗。";
        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;
    }
  }