微信公众号开发时出现40029 错误，不合法的oauth_code该怎么解决呢？用的是c#。

#region 通过code换取openid

    /// <summary>

    /// 根据code获取用户的openid

    /// </summary>

    /// <param name="code"></param>

    /// <returns></returns>      

    public static string CodeByOpenid(string code)

    {

        Log("0");

        string OpenID = string.Empty;

        string getUrl = "https://api.weixin.qq.com/sns/oauth2/access_token?appid={0}&secret={1}&code={2}&grant_type=authorization_code";

        getUrl = string.Format(getUrl, APPID, AppSecret, code);

        Uri uri = new Uri(getUrl);

        HttpWebRequest webReq = (HttpWebRequest)WebRequest.Create(uri);

        webReq.Method = "POST";

        //获取返回信息

        Log("1");

        HttpWebResponse response = (HttpWebResponse)webReq.GetResponse();

        StreamReader streamReader = new StreamReader(response.GetResponseStream(), Encoding.Default);

        string returnJason = streamReader.ReadToEnd();

        Log(returnJason);

        JavaScriptSerializer serializer = new JavaScriptSerializer();

        Dictionary<string, object> json = (Dictionary<string, object>)serializer.DeserializeObject(returnJason);

        object value;

        if (json.TryGetValue("openid", out value))

        {

            OpenID = value.ToString();

        }

        return OpenID;

    }

    #endregion