代币C#的Google Auth Exchange代码

伙计们当我发出这个http请求时，我得到了一个错误

远程服务器返回错误：(400(错误请求

        try
        {
            var request = (HttpWebRequest)WebRequest.Create("https://accounts.google.com/o/oauth2/token");
            string postData = string.Format("code={0}&client_id={1}&client_secret={2}&redirect_uri={3}&grant_type=authorization_code", authCode, clientid, secret, redirectURI);
            var data = Encoding.ASCII.GetBytes(postData);
            request.Method = "POST";
            request.ContentType = "application/x-www-form-urlencoded";
            request.ContentLength = data.Length;
            using (var stream = request.GetRequestStream())
            {
                stream.Write(data, 0, data.Length);
            }
            var response = (HttpWebResponse)request.GetResponse();
            var responseString = new StreamReader(response.GetResponseStream()).ReadToEnd();
            MessageBox.Show(responseString);

        }
        catch (WebException ex)
        {
            MessageBox.Show(ex.Message);
            return null;
        }
    }

我尝试了很多东西，比如：其他链接请求，UrlEncode，将响应放入using((中，但仍然是这个糟糕的请求。重点是从响应中获取accces令牌