How to get access token for google oauth?

My code is working, I have done mistakes in above two lines. It should be like this

byte[] buffer = Encoding.ASCII.GetBytes("code=" + code + "&client_id=xxx&client_secret=xxx&redirect_uri=xxxx&grant_type=authorization_code");
HttpWebRequest req = (HttpWebRequest)WebRequest.Create("https://accounts.google.com/o/oauth2/token");

Remaining code is correct.

As I had similar problems in the process of implementing Google auth, I will post the code that works.. The last mentioned problem: error (400) Bad request could be caused by leading '?' in the above code..

 string codeClient = "code="+ t +"&client_id=number.apps.googleusercontent.com&";
 string secretUri = "client_secret=yoursecret&" + "redirect_uri=path&"
      + "grant_type=authorization_code";
 postString = codeClient + secretUri;

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

 HttpWebRequest request = (HttpWebRequest) WebRequest.Create(url.ToString());
 request.Method = "POST";
 request.ContentType = "application/x-www-form-urlencoded";

 UTF8Encoding utfenc = new UTF8Encoding();
 byte[] bytes = utfenc.GetBytes(postString);
 Stream os = null;
 try
 {
      request.ContentLength = bytes.Length;
      os = request.GetRequestStream();
      os.Write(bytes, 0, bytes.Length);
 }
 catch
 { }

 try
 {
      HttpWebResponse webResponse = (HttpWebResponse) request.GetResponse();
      Stream responseStream = webResponse.GetResponseStream();
      StreamReader responseStreamReader = new StreamReader(responseStream);
      result = responseStreamReader.ReadToEnd();//parse token from result

I think you are sending the POST request to the wrong endpoint, the correct one is https://accounts.google.com/o/oauth2/token