Use google credentials to login into UWP C# app

Taking a look at Google's GitHub it seems that .Net API is still not ready for UWP (however if you traverse the issues you will find that they are working on it, so it's probably a matter of time when official version is ready and this answer would be obsolete).

As I think getting simple accessToken (optionaly refresing it) to basic profile info should be sufficient for this case. Basing on available samples from Google I've build a small project (source at GitHub), that can help you.

So first of all you have to define your app at Google's developer console and obtain ClientID and ClientSecret. Once you have this you can get to coding. To obtain accessToken I will use a WebAuthenticationBroker:

string authString = "https://accounts.google.com/o/oauth2/auth?client_id=" + ClientID;
authString += "&scope=profile";
authString += $"&redirect_uri={RedirectURI}";
authString += $"&state={state}";
authString += $"&code_challenge={code_challenge}";
authString += $"&code_challenge_method={code_challenge_method}";
authString += "&response_type=code";

var receivedData = await WebAuthenticationBroker.AuthenticateAsync(WebAuthenticationOptions.UseTitle, new Uri(authString), new Uri(ApprovalEndpoint));

switch (receivedData.ResponseStatus)
{
    case WebAuthenticationStatus.Success:
        await GetAccessToken(receivedData.ResponseData.Substring(receivedData.ResponseData.IndexOf(' ') + 1), state, code_verifier);
        return true;
    case WebAuthenticationStatus.ErrorHttp:
        Debug.WriteLine($"HTTP error: {receivedData.ResponseErrorDetail}");
        return false;

    case WebAuthenticationStatus.UserCancel:
    default:
        return false;
}

If everything goes all right and user puts correct credentials, you will have to ask Google for tokens (I assume that you only want the user to put credentials once). For this purpose you have the method GetAccessToken:

// Parses URI params into a dictionary - ref: http://stackoverflow.com/a/11957114/72176 
Dictionary<string, string> queryStringParams = data.Split('&').ToDictionary(c => c.Split('=')[0], c => Uri.UnescapeDataString(c.Split('=')[1]));

StringContent content = new StringContent($"code={queryStringParams["code"]}&client_secret={ClientSecret}&redirect_uri={Uri.EscapeDataString(RedirectURI)}&client_id={ClientID}&code_verifier={codeVerifier}&grant_type=authorization_code",
                                          Encoding.UTF8, "application/x-www-form-urlencoded");

HttpResponseMessage response = await httpClient.PostAsync(TokenEndpoint, content);
string responseString = await response.Content.ReadAsStringAsync();

if (!response.IsSuccessStatusCode)
{
    Debug.WriteLine("Authorization code exchange failed.");
    return;
}

JsonObject tokens = JsonObject.Parse(responseString);
accessToken = tokens.GetNamedString("access_token");

foreach (var item in vault.RetrieveAll().Where((x) => x.Resource == TokenTypes.AccessToken.ToString() || x.Resource == TokenTypes.RefreshToken.ToString())) vault.Remove(item);

vault.Add(new PasswordCredential(TokenTypes.AccessToken.ToString(), "MyApp", accessToken));
vault.Add(new PasswordCredential(TokenTypes.RefreshToken.ToString(), "MyApp", tokens.GetNamedString("refresh_token")));
TokenLastAccess = DateTimeOffset.UtcNow;

Once you have the tokens (I'm saving them in PasswordVault for safety), you can later then use them to authenticate without asking the user for his credentials. Note that accessToken has limited lifetime, therefore you use refreshToken to obtain a new one:

if (DateTimeOffset.UtcNow < TokenLastAccess.AddSeconds(3600))
{
    // is authorized - no need to Sign In
    return true;
}
else
{
    string token = GetTokenFromVault(TokenTypes.RefreshToken);
    if (!string.IsNullOrWhiteSpace(token))
    {
        StringContent content = new StringContent($"client_secret={ClientSecret}&refresh_token={token}&client_id={ClientID}&grant_type=refresh_token",
                                                  Encoding.UTF8, "application/x-www-form-urlencoded");

        HttpResponseMessage response = await httpClient.PostAsync(TokenEndpoint, content);
        string responseString = await response.Content.ReadAsStringAsync();

        if (response.IsSuccessStatusCode)
        {
            JsonObject tokens = JsonObject.Parse(responseString);

            accessToken = tokens.GetNamedString("access_token");

            foreach (var item in vault.RetrieveAll().Where((x) => x.Resource == TokenTypes.AccessToken.ToString())) vault.Remove(item);

            vault.Add(new PasswordCredential(TokenTypes.AccessToken.ToString(), "MyApp", accessToken));
            TokenLastAccess = DateTimeOffset.UtcNow;
            return true;
        }
    }
}

The code above is only a sample (with some shortcuts) and as mentioned above - a working version with some more error handling you will find at my GitHub. Please also note, that I haven't spend much time on this and it will surely need some more work to handle all the cases and possible problems. Though hopefully will help you to start.