微博API https://api.weibo.com/oauth2/access_token 报错

我调用微博api的获取access_token接口报错：
{"error":"invalid_grant","error_code":21325,"request":"/oauth2/access_token","error_uri":"/oauth2/access_token","error_description":"invalid authorization code:7abb88384ea0a118814aadde61bdcccb"}
调用方式是：
https://api.weibo.com/oauth2/access_token?client_id=%E8%87%AA%E5%B7%B1%E7%9A%84appkey&client_secret=%E8%87%AA%E5%B7%B1%E7%9A%84appsecret&grant_type=authorization_code&redirect_uri=%E5%9B%9E%E8%B0%83%E5%9C%B0%E5%9D%80&code=7abb88384ea0a118814aadde61bdcccb

这到底是为什么，是因为code值不能重复使用吗，还是其它原因，我这是在postman上面测试的，用HttpWebRequest结果是一直报400服务器错误，我c#是这样调用的（我看说明是要使用post，但是值是拼在url上的，body可以不传值，我用postman也是post调用，但是值直接拼在url上，没有写在body中，能调但是报上面错）：



 public static string HttpPost(string Url, string postDataStr)
        { 
                int I = 0; 
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Url);
                CookieContainer cookieCon = new CookieContainer(); 
                request.Method = "POST";
                request.Timeout = 50000;  
                request.CookieContainer = cookieCon;
                request.KeepAlive = true;
                request.ProtocolVersion = HttpVersion.Version10; 
                request.ServicePoint.Expect100Continue = false; 
                Stream myRequestStream = request.GetRequestStream();
                StreamWriter myStreamWriter = new StreamWriter(myRequestStream);
                myStreamWriter.Write(postDataStr);
                myStreamWriter.Close();
                string retString = null; 
                HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                if (response != null)
                {
                    response.Cookies = cookieCon.GetCookies(response.ResponseUri);
                    Stream myResponseStream = response.GetResponseStream();
                    StreamReader myStreamReader = new StreamReader(myResponseStream, Encoding.GetEncoding("utf-8"));
                    retString = myStreamReader.ReadToEnd();
                    myStreamReader.Close();
                    myResponseStream.Close();
                    myResponseStream.Dispose(); 
                    myRequestStream.Close();
                    myRequestStream.Dispose();
                }

                return retString; 

        }