C#/ASP.NET/ASP MVC - Send email using Google/Gmail OAuth 2.0 authentication¶

using System;
using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.InteropServices;
using EASendMail;

namespace EASendMailKbCSharp
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("+------------------------------------------------------------------+");
            Console.WriteLine("  Sign in with Google                                             ");
            Console.WriteLine("   If you got \"This app isn't verified\" information in Web Browser, ");
            Console.WriteLine("   click \"Advanced\" -> Go to ... to continue test.");
            Console.WriteLine("+------------------------------------------------------------------+");
            Console.WriteLine("");
            Console.WriteLine("Press any key to sign in...");
            Console.ReadKey();

            try
            {
                Program p = new Program();
                p.DoOauthAndSendEmail();
            }
            catch (Exception ep)
            {
                Console.WriteLine(ep.ToString());
            }

            Console.ReadKey();
        }

        void SendMailWithXOAUTH2(string userEmail, string accessToken)
        {
            // Gmail SMTP server address
            SmtpServer oServer = new SmtpServer("smtp.gmail.com");
            // enable SSL connection
            oServer.ConnectType = SmtpConnectType.ConnectSSLAuto;
            // Using 587 port, you can also use 465 port
            oServer.Port = 587;

            // use Gmail SMTP OAUTH 2.0 authentication
            oServer.AuthType = SmtpAuthType.XOAUTH2;
            // set user authentication
            oServer.User = userEmail;
            // use access token as password
            oServer.Password = accessToken;

            SmtpMail oMail = new SmtpMail("TryIt");
            // Your Gmail email address
            oMail.From = userEmail;

            // Please change recipient address to yours for test
            oMail.To = "support@emailarchitect.net";

            oMail.Subject = "test email from gmail account with OAUTH 2";
            oMail.TextBody = "this is a test email sent from c# project with gmail.";

            Console.WriteLine("start to send email using OAUTH 2.0 ...");

            SmtpClient oSmtp = new SmtpClient();
            oSmtp.SendMail(oServer, oMail);

            Console.WriteLine("The email has been submitted to server successfully!");
        }

        // client configuration
        // You should create your client id and client secret,
        // do not use the following client id in production environment, it is used for test purpose only.
        const string clientID = "507778793027-cfod2rvnnks95ed17d7ejepp16l1tvg8.apps.googleusercontent.com";
        const string clientSecret = "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";
        const string scope = "openid%20profile%20email%20https://mail.google.com";
        const string authUri = "https://accounts.google.com/o/oauth2/auth";
        const string tokenUri = "https://oauth2.googleapis.com/token";

        static int GetRandomUnusedPort()
        {
            var listener = new TcpListener(IPAddress.Loopback, 0);
            listener.Start();
            var port = ((IPEndPoint)listener.LocalEndpoint).Port;
            listener.Stop();
            return port;
        }

        async void DoOauthAndSendEmail()
        {
            // Creates a redirect URI using an available port on the loopback address.
            string redirectUri = string.Format("http://127.0.0.1:{0}/", GetRandomUnusedPort());
            Console.WriteLine("redirect URI: " + redirectUri);

            // Creates an HttpListener to listen for requests on that redirect URI.
            var http = new HttpListener();
            http.Prefixes.Add(redirectUri);
            Console.WriteLine("Listening ...");
            http.Start();

            // Creates the OAuth 2.0 authorization request.
            string authorizationRequest = string.Format("{0}?response_type=code&scope={1}&redirect_uri={2}&client_id={3}",
                authUri,
                scope,
                Uri.EscapeDataString(redirectUri),
                clientID
            );

            // Opens request in the browser.
            System.Diagnostics.Process.Start(new System.Diagnostics.ProcessStartInfo(authorizationRequest) { UseShellExecute = true });

            // Waits for the OAuth authorization response.
            var context = await http.GetContextAsync();

            // Brings the Console to Focus.
            BringConsoleToFront();

            // Sends an HTTP response to the browser.
            var response = context.Response;
            string responseString = string.Format("<html><head></head><body>Please return to the app and close current window.</body></html>");
            var buffer = Encoding.UTF8.GetBytes(responseString);
            response.ContentLength64 = buffer.Length;
            var responseOutput = response.OutputStream;
            Task responseTask = responseOutput.WriteAsync(buffer, 0, buffer.Length).ContinueWith((task) =>
            {
                responseOutput.Close();
                http.Stop();
                Console.WriteLine("HTTP server stopped.");
            });

            // Checks for errors.
            if (context.Request.QueryString.Get("error") != null)
            {
                Console.WriteLine(string.Format("OAuth authorization error: {0}.", context.Request.QueryString.Get("error")));
                return;
            }

            if (context.Request.QueryString.Get("code") == null)
            {
                Console.WriteLine("Malformed authorization response. " + context.Request.QueryString);
                return;
            }

            // extracts the code
            var code = context.Request.QueryString.Get("code");
            Console.WriteLine("Authorization code: " + code);

            string responseText = await RequestAccessToken(code, redirectUri);
            Console.WriteLine(responseText);

            OAuthResponseParser parser = new OAuthResponseParser();
            parser.Load(responseText);

            var user = parser.EmailInIdToken;
            var accessToken = parser.AccessToken;

            Console.WriteLine("User: {0}", user);
            Console.WriteLine("AccessToken: {0}", accessToken);

            SendMailWithXOAUTH2(user, accessToken);
        }

        async Task<string> RequestAccessToken(string code, string redirectUri)
        {
            Console.WriteLine("Exchanging code for tokens...");

            // builds the  request
            string tokenRequestBody = string.Format("code={0}&redirect_uri={1}&client_id={2}&client_secret={3}&grant_type=authorization_code",
                code,
                Uri.EscapeDataString(redirectUri),
                clientID,
                clientSecret
                );

            // sends the request
            HttpWebRequest tokenRequest = (HttpWebRequest)WebRequest.Create(tokenUri);
            tokenRequest.Method = "POST";
            tokenRequest.ContentType = "application/x-www-form-urlencoded";
            tokenRequest.Accept = "Accept=text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";

            byte[] _byteVersion = Encoding.ASCII.GetBytes(tokenRequestBody);
            tokenRequest.ContentLength = _byteVersion.Length;

            Stream stream = tokenRequest.GetRequestStream();
            await stream.WriteAsync(_byteVersion, 0, _byteVersion.Length);
            stream.Close();

            try
            {
                // gets the response
                WebResponse tokenResponse = await tokenRequest.GetResponseAsync();
                using (StreamReader reader = new StreamReader(tokenResponse.GetResponseStream()))
                {
                    // reads response body
                    return await reader.ReadToEndAsync();
                }

            }
            catch (WebException ex)
            {
                if (ex.Status == WebExceptionStatus.ProtocolError)
                {
                    var response = ex.Response as HttpWebResponse;
                    if (response != null)
                    {
                        Console.WriteLine("HTTP: " + response.StatusCode);
                        using (StreamReader reader = new StreamReader(response.GetResponseStream()))
                        {
                            // reads response body
                            string responseText = await reader.ReadToEndAsync();
                            Console.WriteLine(responseText);
                        }
                    }
                }

                throw ex;
            }
        }

        // Hack to bring the Console window to front.

        [DllImport("kernel32.dll", ExactSpelling = true)]
        public static extern IntPtr GetConsoleWindow();

        [DllImport("user32.dll")]
        [return: MarshalAs(UnmanagedType.Bool)]
        public static extern bool SetForegroundWindow(IntPtr hWnd);

        public void BringConsoleToFront()
        {
            SetForegroundWindow(GetConsoleWindow());
        }
    }
}

// Please add http://localhost:54098/oauth/token to Authorized redirect URIs in your Google/MS Azure project.
public ActionResult Token(string code)
{
    // code parameter is the authorization code returned by Google OAuth server,
    // then you can use it to request AccessToken
    // just like RequestAccessToken method in previous example
}