How to get access token for google oauth?

Question

I am using C# (ASP.NET). I want to use Google OAuth for accessing the user profile detail in my app. I successfully got the authorization code but having a problem in getting the access token. I prefer the Google tutorials. In tutorial, I read that I have to send the request and get the response from google. For that, I use System.Net.HttpWebRequest/HttpWebResponse (am I going in the right way). I have used this code...

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("http://accounts.google.com");
req.Method = "POST";
req.ContentType = "application/x-www-form-urlencoded";
req.ContentLength = buffer.Length;

Stream strm = req.GetRequestStream();
strm.Write(buffer, 0, buffer.Length);
strm.Close();

HttpWebResponse resp = (HttpWebResponse)req.GetResponse();
Response.Write(((HttpWebResponse)resp).StatusDescription);

But, I got the error:

The remote server returned an error: (405) Method Not Allowed.

Update: Here variable code is authorization code.

Answer 1

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

Answer 2

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