Login using Google OAuth 2.0 with C#

I am explaining based on Google+ API, which uses Gmail ID to login. So, you will be authenticating your users to login with Gmail.

1: You need to turn on the Google+ API:

2: Once you turned on the Google+ API, then you need to add new Client ID.

Step 2

Step 3

Here in Step 2, when you add Redirect URL, you will need to add the URL of your website on which page you want user to redirected to.

Once you have created your Client ID for Web Application.

Then in your application, you need to add two packages

1: Newtonsoft.Json

2: Microsoft.Net.Http

Now add this namespaces;

using Newtonsoft.Json;
using System.IO;
using System.Net;
using System.Net.Http;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

Now in code first your declare this variables at top of your page;

protected string googleplus_client_id = "458878619548-khuatamj3qpiccnsm4q6dbulf13jumva.apps.googleusercontent.com";    // Replace this with your Client ID
protected string googleplus_client_secret = "4hiVJYlomswRd_PV5lyNQlfN";                                                // Replace this with your Client Secret
protected string googleplus_redirect_url = "http://localhost:2443/Index.aspx";                                         // Replace this with your Redirect URL; Your Redirect URL from your developer.google application should match this URL.
protected string Parameters;

Then in you Page Load event;

protected void Page_Load(object sender, EventArgs e)
{
    if ((Session.Contents.Count > 0) && (Session["loginWith"] != null) && (Session["loginWith"].ToString() == "google"))
    {
        try
        {
            var url = Request.Url.Query;
            if (url != "")
            {
                string queryString = url.ToString();
                char[] delimiterChars = { '=' };
                string[] words = queryString.Split(delimiterChars);
                string code = words[1];

                if (code != null)
                {
                    //get the access token 
                    HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create("https://accounts.google.com/o/oauth2/token");
                    webRequest.Method = "POST";
                    Parameters = "code=" + code + "&client_id=" + googleplus_client_id + "&client_secret=" + googleplus_client_secret + "&redirect_uri=" + googleplus_redirect_url + "&grant_type=authorization_code";
                    byte[] byteArray = Encoding.UTF8.GetBytes(Parameters);
                    webRequest.ContentType = "application/x-www-form-urlencoded";
                    webRequest.ContentLength = byteArray.Length;
                    Stream postStream = webRequest.GetRequestStream();
                    // Add the post data to the web request
                    postStream.Write(byteArray, 0, byteArray.Length);
                    postStream.Close();

                    WebResponse response = webRequest.GetResponse();
                    postStream = response.GetResponseStream();
                    StreamReader reader = new StreamReader(postStream);
                    string responseFromServer = reader.ReadToEnd();

                    GooglePlusAccessToken serStatus = JsonConvert.DeserializeObject<GooglePlusAccessToken>(responseFromServer);

                    if (serStatus != null)
                    {
                        string accessToken = string.Empty;
                        accessToken = serStatus.access_token;

                        if (!string.IsNullOrEmpty(accessToken))
                        {
                            // This is where you want to add the code if login is successful.
                            // getgoogleplususerdataSer(accessToken);
                        }
                    }

                }
            }
        }
        catch (Exception ex)
        {
            //throw new Exception(ex.Message, ex);
            Response.Redirect("index.aspx");
        }
    }
}

Now the event that will call the google API

protected void Google_Click(object sender, EventArgs e)
{
     var Googleurl = "https://accounts.google.com/o/oauth2/auth?response_type=code&redirect_uri=" + googleplus_redirect_url + "&scope=https://www.googleapis.com/auth/userinfo.email%20https://www.googleapis.com/auth/userinfo.profile&client_id=" + googleplus_client_id;
     Session["loginWith"] = "google";
     Response.Redirect(Googleurl);
}

Add this GooglePlusAccessToken class;

// Google
public class GooglePlusAccessToken
{
    public string access_token { get; set; }
    public string token_type { get; set; }
    public int expires_in { get; set; }
    public string id_token { get; set; }
    public string refresh_token { get; set; }
}

Also you can call other oauth API with the Access Token to retrieve some of users information.

private async void getgoogleplususerdataSer(string access_token)
{
    try
    {
        HttpClient client = new HttpClient();
        var urlProfile = "https://www.googleapis.com/oauth2/v1/userinfo?access_token=" + access_token;

        client.CancelPendingRequests();
        HttpResponseMessage output = await client.GetAsync(urlProfile);

        if (output.IsSuccessStatusCode)
        {
            string outputData = await output.Content.ReadAsStringAsync();
            GoogleUserOutputData serStatus = JsonConvert.DeserializeObject<GoogleUserOutputData>(outputData);

            if (serStatus != null)
            {
                 // You will get the user information here.
            }
        }
    }
    catch (Exception ex)
    { 
         //catching the exception
    }
}

public class GoogleUserOutputData
{
    public string id { get; set; }
    public string name { get; set; }
    public string given_name { get; set; }
    public string email { get; set; }
    public string picture { get; set; }
}

Hope this is what you were looking for, I implemented this and it is working just fine.
Hope this helps.