前端通过调用地址获取到code,

window.location.href = `https://open.weixin.qq.com/connect/oauth2/authorize?appid=${user_appid}& redirect_uri=${encodeURIComponent('http://192.168.3.189/h5/pages/login/loginByAccount/getUserAccount')}&response_type=code&scope=snsapi_base&state=STATE&connect_redirect=1#wechat_redirect`;

然后我提交code提到到服务端获取openid

后端服务代码

 public static string GetOpenIdUrl(string appId, string secret, string code, string grantType = "authorization_code")
        {
            object[] args = new object[] { appId, secret, code, grantType };
            var url = string.Format("https://api.weixin.qq.com/sns/oauth2/access_token?appid={0}&secret={1}&code={2}&grant_type={3}",args);
            return url;


        }


        /// <summary>
        /// 获取openid
        /// </summary>
        /// <param name="appId"></param>
        /// <param name="secret"></param>
        /// <param name="code"></param>
        /// <param name="grantType"></param>
        /// <returns></returns>
        public static string GetOpenid(string appId, string secret, string code, string grantType = "authorization_code")
        {
            string requestUri = GetOpenIdUrl(appId, secret, code, grantType);
            var responseStr = httpClient.GetAsync(requestUri).Result.Content.ReadAsStringAsync().Result;
            var obj = JsonConvert.DeserializeObject<Dictionary<string, string>>(responseStr);
            string openid = string.Empty;
            if (!obj.TryGetValue("openid", out openid))
            {
                //Log.Info($"获取openid失败appId={appId}，secret={secret}，code={code}");
            }
            return openid;
        }

但我后端反回40029错误代码，请问要怎么才能得到正确的结果？