Can't get a token from the Google API - authentication

Question

I'm trying to exchange my authorization code for a token with the Google OAuth2 API for my Windows 8 app, but I keep getting HTTP 400 errors.
This is how I perform the request (simplified):
var url = "https://accounts.google.com/o/oauth2/token";
var body = "code=4/LEXF1iAVRZvfCfdQg9r1aFqoYDgV&client_id=904019870963.apps.googleusercontent.com&client_secret=[removed]&redirect_uri=urn:ietf:wg:oauth:2.0:oob&grant_type=authorization_code";
HttpClient httpClient = new HttpClient();
HttpResponseMessage response = await httpClient.PostAsync(new Uri(url), new StringContent(body));
response.EnsureSuccessStatusCode();
Visual Studio usually simply gives me a HTTP 400 bad request error, when I try the same thing in Fiddler I also get an HTTP 400 error, but with this as content:
21
{
"error" : "invalid_request"
}
0
I read all the documentation about Google OAuth, I searched on Google and StackOverflow for this issue, I tried changing all different sorts of things in my code (UrlEncode, etc.), I used the Google API Playground to see what kind of requests it performs and compared it with my own requests (couldn't find a difference except for the return URL, the auth code and the user-agent). No matter what I do, I just can't get this working, I've been stuck for hours already.
Can anyone help me out here?

Answer 1

Read the body content to get the error json you noticed in Fiddler.
HttpClient httpClient = new HttpClient();
var response = httpClient.PostAsync(new Uri(url), new StringContent(body)).Result;
var content = response.Content.ReadAsStringAsync().Result;
content now holds :
{
"error" : "invalid_request"
}
You could project the error in an object, by specifing your error type like: response.Content.ReadAsAsync().Result
For the invalid request part, you should play with UrlEncode. I know you say you tried it, but applying it at the right spots really solves your problem.
var body = "code="+WebUtility.UrlEncode("4/LEXF1iAVRZvfCfdQg9r1aFqoYDgV")+
"&redirect_uri="+WebUtility.UrlEncode("https://yoursite...")+
"&client_id=904019870963.apps.googleusercontent.com" +
"&scope=" +
"&client_secret=********" +
"&grant_type=authorization_code";
HttpClient httpClient = new HttpClient();
var response = httpClient.PostAsync(new Uri(endpoint),
new StringContent(body, Encoding.UTF8, "application/x-www-form-urlencoded")).Result;
This is the code that works for me. Should work for you also.