Retrieve OpenID via OAuth2 Authorization

To determine whether a user has subscribed to a specific WeChat official account, the first step is obtaining their OpenID. This can be acheived using WeChat's OAuth2 authorization flow with minimal scope (snsapi_base), which does not require explicit user consent for basic information.

#region Obtain WeChat Payment OpenID

/// <summary>
/// Retrieves the user's OpenID through OAuth2 authorization.
/// </summary>
/// <returns>User's OpenID or empty string on failure</returns>
public string GetPayOpenID()
{
    try
    {
        string code = Request.QueryString["code"];
        
        if (string.IsNullOrEmpty(code))
        {
            // Redirect to WeChat OAuth2 authorization page
            string redirectUrl = Request.Url.ToString();
            string authorizationUrl = 
                "https://open.weixin.qq.com/connect/oauth2/authorize?" +
                "appid=" + Constant.WX_PAY_APPID +
                "&redirect_uri=" + HttpUtility.UrlEncode(redirectUrl) +
                "&response_type=code" +
                "&scope=snsapi_base" +
                "&state=123#wechat_redirect";

            Response.Redirect(authorizationUrl);
            return null;
        }
        else
        {
            // Exchange code for access token and retrieve OpenID
            string accessTokenUrl = 
                "https://api.weixin.qq.com/sns/oauth2/access_token?" +
                "appid=" + Constant.WX_PAY_APPID +
                "&secret=" + Constant.WX_PAY_APPSECRET +
                "&code=" + code +
                "&grant_type=authorization_code";

            var httpClient = new System.Net.Http.HttpClient();
            string jsonResponse = httpClient.GetStringAsync(accessTokenUrl).Result;

            JObject tokenData = JObject.Parse(jsonResponse);
            string openId = tokenData["openid"]?.ToString();
            return openId;
        }
    }
    catch (Exception ex)
    {
        // Log exception as needed
        return string.Empty;
    }
}

#endregion

Acquire Access Token for API Calls

Prior to querying user subscription status, your server must obtain a valid access_token by calling the client credential endpoint. This token grants temporary access to WeChat's backend APIs.

GET https://api.weixin.qq.com/cgi-bin/token?
  grant_type=client_credential&appid=YOUR_APPID&secret=YOUR_SECRET

Successful response:

{
    "access_token": "O3PwqnKoiiYmsHeAh8viWLQyhGRrGU6RT9o53pvlmhIBWQnTXeZDSkYNw6YufzIDUspzQguvxtmLXtAWmQd2NmurXKa4N4PsbwG7RvI25pqzSC3-cLl50iqSW5VaZ4xmGXQgAFAJAT",
    "expires_in": 7200
}

Query User Subscripsion Status Using OpenID

With both the access_token and openid, you can now call the user info API to check subscription status.

GET https://api.weixin.qq.com/cgi-bin/user/info?
  access_token=ACCESS_TOKEN&openid=OPENID&lang=zh_CN

Sample resposne when user is subscribed:

{
    "subscribe": 1,
    "openid": "oeQDZt0n4VCZ70wy***",
    "nickname": "背上***旅行",
    "sex": 1,
    "language": "zh_CN",
    "city": "昌平",
    "province": "北京",
    "country": "中国",
    "headimgurl": "http://wx.qlogo.cn/mmopen/kBwGJuwqK9**********************ibVUEpgFE90LH3b3uj7AYRjZP/0",
    "subscribe_time": 1474964999,
    "unionid": "oGCG8t5**********jPQTPw",
    "remark": "",
    "groupid": 0,
    "tagid_list": []
}

Response when user is not subscribed:

{
    "subscribe": 0,
    "openid": "oeQDZtzPrIYwOPXdzgKmd0gYPG44",
    "unionid": "oGCG8tzd2NwsOYAeWAZMwV9rMmPU",
    "tagid_list": []
}

The key field is subscribe:
- 1 indicates the user follows the official account.
- 0 means they do not subscribe, and only limited data (like openid and unionid) will be returned.