c# – google api oauth访问令牌检索 – 400个错误请求

public ActionResult GetOAuthToken()
        {

        String url = "https://accounts.google.com/o/oauth2/token";

        // Create a request using a URL that can receive a post. 
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
        // Set the Method property of the request to POST.
        request.Method = "POST";
        request.Host = "accounts.google.com";
        // Create POST data and convert it to a byte array.
        string postData = String.Format("code={0}&client_id={1}&client_secret={2}&redirect_uri={3}&grant_type=authorization_code",Request.QueryString["code"].ToString(),OAuthConfig.client_id,OAuthConfig.client_secret,OAuthConfig.token_redirect_uri);
        ASCIIEncoding encoding = new ASCIIEncoding();
        byte[] byteArray = encoding.GetBytes(postData);
        // Set the ContentType property of the WebRequest.
        request.ContentType = "application/x-www-form-urlencoded";
        // Set the ContentLength property of the WebRequest.
        request.ContentLength = byteArray.Length;
        // Get the request stream.
        Stream dataStream = request.GetRequestStream();

        // Write the data to the request stream.
        dataStream.Write(byteArray,byteArray.Length);
        // Close the Stream object.
        dataStream.Close();
        // Get the response.
        WebResponse response = request.GetResponse();

        // SOME CODE TO PROCESS THE RESPONSE

        Response.Redirect("/Home");
        return View();
        }

OAuthConfig只是一个包含客户端ID,客户端密钥等的类.

我一直得到’远程服务器返回错误：(400)错误请求.在request.GetResponse()行.我哪里错了？