作者：奋斗放个，原文地址

一：客户端

    var that = this
    wx.login({
      success: function (res) {
        var appId = '     ';//微信公众号平台申请的appid
        var appSecret = '   ';//微信公众号平台申请的app secret
        var js_code = res.code;//调用登录接口获得的用户的登录凭证code
        wx.request({
          url: ' + appId + '&secret=' + appSecret + '&js_code=' + js_code + '&grant_type=authorization_code',
          data: {},
          method: 'GET',
          success: function (res) {
            var openid = res.data.openid //返回的用户唯一标识符openid
            console.log(openid)
            console.log("试试吧上面就是获得的openid")
          }
        })
      }
    })
    //试验自己的服务器获取openId
    //调用登录接口
    wx.login({
      success: function (res) {
        //console.log(res);
        that.globalData.loginCode = res.code
        wx.getUserInfo({
          success: function (res) {
            that.globalData.userInfo = res.userInfo
            that.globalData.iv = res.iv
            that.globalData.encryptedData = res.encryptedData
            typeof cb == "function" && cb(that.globalData.userInfo)
            that.req(
              'https://lifar网址.aspx?Action=ActionLogin',
              {
                encryptedData: that.globalData.encryptedData,
                iv: that.globalData.iv,
                code: that.globalData.loginCode
              },
              'GET',
              function (res) {
                console.log(res)
                if (res.data.success) {
                  console.log('试验自己的服务器获取openId:')
                  console.log(res.data)
                  var sessionId = res.data.result;
                  wx.setStorageSync('sessionId', sessionId)
                  console.log(sessionId)
                }
              },
              function (res) {
                console.log(res)
              }
            );
          }
        })
      }
    })
    //试验自己的服务器获取openId结束

方法：

  req: function (url, data, method, success, fail) {
    var mydata = data || {};
    //mydata['appId'] = app.globalData.appId;
    wx.request({
      url: url,
      data: mydata,
      method: method,
      success: success,
      fail: fail,
      complete: function () {
        // complete
      }
    })
  }

二：服务端

        string JsCode2SessionUrl = "";
        protected void Page_Load(object sender, EventArgs e)
        {
            JsCode2SessionUrl = "{0}&secret={1}&js_code={2}&grant_type=authorization_code";
            if (Request["Action"] == "ActionLogin")
            {
                string str_encryptedData = Request["encryptedData"];
                string iv = Request["iv"];
                //小程序appid和appsecret配置
                string appid = Request["appid"];
                string secret = Request["secret"];
                //如果不传过来可以在web.config里配置
                appid = ConfigurationManager.AppSettings["wx_appid"];
                secret = ConfigurationManager.AppSettings["wx_key"];
                string code = Request["code"];
                string sessionId = JsCode2Session(appid, secret, code);
                //Session["sessionId"] = sessionId;
                if (sessionId == "")
                {
                    Response.Write("{\"result\":\"session_key和openid取不到\",\"success\":false}");
                    Response.End();
                }
                string session_key = sessionId.Split(new Char[] { '#' })[0];
                string openid = sessionId.Split(new Char[] { '#' })[1];
                //Response.Write(sessionId);
                Response.Write("{\"result\":\"" + openid + "\",\"success\":true}");
                Response.End();
            }
        }
        //code换取session_key,openid
        //<summary>
        //code换取session_key,openid
        //</summary>
        //<param name="appid"></param>
        //<param name="secret"></param>
        //<param name="code"></param>
        //<returns></returns>
        public string JsCode2Session(string appid, string secret, string code)
        {
            var url = string.Format(JsCode2SessionUrl, appid, secret, code);
            var str = GetFunction(url);
            try
            {
                JsonData jo = JsonMapper.ToObject(str);
                string session_key = jo["session_key"].ToString();
                string weixinID = jo["openid"].ToString();
                return session_key + "#" + weixinID;
            }
            catch (Exception ex)
            {
                return "";
            }
        }
        public string GetFunction(string url)
        {
            string serviceAddress = url;
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(serviceAddress);
            request.Method = "GET";
            request.ContentType = "textml;charset=UTF-8";
            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
            Stream myResponseStream = response.GetResponseStream();
            StreamReader myStreamReader = new StreamReader(myResponseStream, Encoding.UTF8);
            string retString = myStreamReader.ReadToEnd();
            myStreamReader.Close();
            myResponseStream.Close();
            //Response.Write(retString);
            return retString;
        }

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