Managed C++/CLI - Send Email using Google/Gmail OAuth 2.0 Authentication

By default, you need to enable ” Allowing less secure apps” in Gmail, then you can send email with user/password SMTP authentication.

However Google will disable traditional user authentication in the future, switching to Google OAuth is strongly recommended now.

Installation

Before you can use the following codes, please download EASendMail SMTP Component and install it on your machine at first. Full sample proejcts are included in this installer.

Install from NuGet

You can also install the run-time assembly by NuGet. Run the following command in the NuGet Package Manager Console:

Install-Package EASendMail

Note

If you install it by NuGet, no sample projects are installed, only .NET assembly is installed.

Add Reference

To use EASendMail SMTP Component in your project, the first step is Add reference of EASendMail to your project. Please create or open your project with Visual Studio, then go to menu -> Project -> Add Reference -> .NET -> Browse..., and select Installation Path\Lib\net[version]\EASendMail.dll from your disk, click Open -> OK, the reference of EASendMail will be added to your project, and you can start to use it to send email in your project.

add reference in c#/vb.net/c++/cli/clr

.NET Assembly

Because EASendMail has separate builds for .Net Framework, please refer to the following table and choose the correct dll.

Separate builds of run-time assembly for .NET Framework 1.1, 2.0, 3.5, 4.0, 4.5, 4.6.1, .NET Core 2.1 and .NET Compact Framework 2.0, 3.5.

File .NET Framework Version
Lib\net11\EASendMail.dll Built with .NET Framework 1.1
It requires .NET Framework 1.1, 2.0, 3.5 or later version.
Lib\net20\EASendMail.dll Built with .NET Framework 2.0
It requires .NET Framework 2.0, 3.5 or later version.
Lib\net35\EASendMail.dll Built with .NET Framework 3.5
It requires .NET Framework 3.5 or later version.
Lib\net40\EASendMail.dll Built with .NET Framework 4.0
It requires .NET Framework 4.0 or later version.
Lib\net45\EASendMail.dll Built with .NET Framework 4.5
It requires .NET Framework 4.5 or later version.
Lib\net461\EASendMail.dll Built with .NET Framework 4.6.1
It requires .NET Framework 4.6.1 or later version.
Lib\netcoreapp2.1\EASendMail.dll Built with .NET Core 2.1
It requires .NET Core 2.1 or later version.
Lib\net20-cf\EASendMail.dll Built with .NET Compact Framework 2.0
It requires .NET Compact Framework 2.0, 3.5 or later version.
Lib\net35-cf\EASendMail.dll Built with .NET Compact Framework 3.5
It requires .NET Compact Framework 3.5 or later version.

The Gmail IMAP and SMTP servers have been extended to support authorization via the industry-standard OAuth 2.0 protocol. Using OAUTH protocol, user can do authentication by Google Web Login instead of inputting user and password directly in application.

Create Project in Google Developers Console

To send email using Gmail OAuth in your application, you should create a project in Google Developers Console like this:

Create Credentials (OAuth Client ID)

  • Click APIs & Services -> Dashboard -> Credentials

    google oauth Credentials
  • Click Credentials -> Create Credentials -> OAuth client ID -> Web application or Other (Desktop Application). It depends on your application type.

    google oauth Credentials
  • Input a name for your application, input your web applicaton url to receive authorization code at Authorized Redirect URIs. Desktop Application doesn’t require this step.

  • Click "Create", you will get client id and client secret

    google oauth client secret

Enable Gmail API

  • Enable Gmail API in "Library" -> Search "Gmail", then click "Gmail API" and enable it.

    enable Gmail API

Edit Scopes

  • Set detail information for your project at "OAuth consent screen" -> "Edit App".

    edit Gmail oauth consent
  • Finally add "https://mail.google.com/" and "../auth/gmail.send" scopes at "OAuth consent screen" -> "Edit App" -> "Scopes for Google API".

    enable Gmail scope

API Scopes

Gmail supports SMTP + OAuth, but the API (https://mail.google.com/) scope is restricted API which requests to have full access to the Gmail account. Restricted API is throttled before your project is authenticated in by Google.

Using less restricted API (https://www.googleapis.com/auth/gmail.send) scope to send email via Gmail server is recommended.

  • If you use Gmail RESTFul API to send email, please only use "../auth/gmail.send" scope to avoid your app throttled;
  • If you use SMTP protocol, you should use https://mail.google.com/ scope.

Authorized Redirect URIs

If you use OAuth in a web application, you should use a web page or controller to get authorization code from Google OAuth Server. So you need to add your page or web application routing path to Authorized Redirect URIs in APIs & Services -> Dashboard -> Credentials -> OAuth 2.0 Client IDs -> Your Client ID.

Authorized Redirect URIs

Use client id and client secret to request access token

You can use client id and client secret to get the user email address and access token like this:

  • Your application uses a web browser/browser control to open Google OAUTH URL;
  • User inputs user and password in Google Web Authentication Page, and then Google returns access token back to your application;
  • Your application uses access token to access Gmail SMTP server.
  • You can find full example codes in EASendMail Installation Path\Samples_{Programming language/Developer Tool}\Oauth project.

Important

You should create your client id and client secret, do not use the client id from example codes in production environment, it is used for test purpose. If you got "This app isn't verified" information, please click "Advanced" -> Go to ... for test.

Managed C++/CLI - Send Email Using Google OAuth + Gmail SMTP Server

Here is a console application which demonstrates how to use Google OAuth to do user authentication and send email.

Note

This sample cannot handle the event of Web Browser is closed by user manually before authentication is completed. You can refer to the better sample project which uses Web Browser Control in EASendMail installation path.

#include "stdafx.h"

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

// Hack to bring the Console window to front.
[DllImport("kernel32.dll", CharSet = CharSet::Ansi)]
extern "C" void* GetConsoleWindow();

[DllImport("user32.dll", CharSet = CharSet::Ansi)]
extern "C" Int32 SetForegroundWindow(void* hWnd);

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

ref class GoogleOauth
{


public:
    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.
        auto http = gcnew 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(authorizationRequest);

        // Waits for the OAuth authorization response.
        auto asyncResult = http->BeginGetContext(nullptr, nullptr);
        while (!asyncResult->AsyncWaitHandle->WaitOne(10))
        {
        }

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

        auto context = http->EndGetContext(asyncResult);

        // Sends an HTTP response to the browser.
        auto response = context->Response;
        String^ responseString = "<html><head></head><body>Please return to the app and close current window.</body></html>";
        auto buffer = Encoding::UTF8->GetBytes(responseString);
        response->ContentLength64 = buffer->Length;

        auto responseOutput = response->OutputStream;
        responseOutput->Write(buffer, 0, buffer->Length);
        responseOutput->Close();
        http->Stop();
        Console::WriteLine("HTTP server stopped.");

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

        if (context->Request->QueryString->Get("code") == nullptr)
        {
            Console::WriteLine("Malformed authorization response. " + context->Request->RawUrl);
            return;
        }

        // extracts the code
        auto code = context->Request->QueryString->Get("code");
        Console::WriteLine("Authorization code: " + code);

        String^ responseText = RequestAccessToken(code, redirectUri);
        Console::WriteLine(responseText);

        OAuthResponseParser^ parser = gcnew OAuthResponseParser();
        parser->Load(responseText);

        auto user = parser->EmailInIdToken;
        auto accessToken = parser->AccessToken;

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

        SendMailWithXOAUTH2(user, accessToken);
    }

private:
    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
        auto 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";

        auto _byteVersion = Encoding::ASCII->GetBytes(tokenRequestBody);
        tokenRequest->ContentLength = _byteVersion->Length;

        Stream^ stream = tokenRequest->GetRequestStream();
        stream->Write(_byteVersion, 0, _byteVersion->Length);
        stream->Close();

        try
        {
            // gets the response
            auto tokenResponse = tokenRequest->GetResponse();

            {
                StreamReader^ reader = gcnew StreamReader(tokenResponse->GetResponseStream());
                // reads response body
                return reader->ReadToEnd();
            }

        }
        catch (WebException ^ex)
        {
            if (ex->Status == WebExceptionStatus::ProtocolError)
            {
                auto response = (HttpWebResponse^)ex->Response;
                if (response != nullptr)
                {
                    Console::WriteLine("HTTP: " + response->StatusCode.ToString());

                    {
                        StreamReader^ reader = gcnew StreamReader(response->GetResponseStream());
                        // reads response body
                        Console::WriteLine(reader->ReadToEnd());
                    }
                }
            }

            throw ex;
        }
    }

    void SendMailWithXOAUTH2(String^ userEmail, String^ accessToken)
    {
        // Gmail SMTP server address
        SmtpServer ^oServer = gcnew 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 = gcnew SmtpMail("TryIt");
        // Your Gmail email address
        oMail->From = userEmail;

        // Please change recipient address to yours for test
        oMail->To = gcnew String("support@emailarchitect.net");

        oMail->Subject = "test email from gmail account with OAUTH 2";
        oMail->TextBody = "this is a test email sent from C++/CLR project with gmail.";

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

        SmtpClient ^oSmtp = gcnew 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.
    String^ clientID = "1072602369179-aru4rj97ateiho9rt4pf5i8l1r01mc16.apps.googleusercontent.com";
    String^ clientSecret = "Lnw8r5FvfKFNS_CSEucbdIE-";
    String^ scope = "openid%20profile%20email%20https://mail.google.com";
    String^ authUri = "https://accounts.google.com/o/oauth2/v2/auth";
    String^ tokenUri = "https://www.googleapis.com/oauth2/v4/token";

    static int GetRandomUnusedPort()
    {
        auto listener = gcnew TcpListener(IPAddress::Loopback, 0);
        listener->Start();
        auto port = ((IPEndPoint^)listener->LocalEndpoint)->Port;
        listener->Stop();
        return port;
    }
};

int main(array<System::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
    {
        GoogleOauth^ p = gcnew GoogleOauth();
        p->DoOauthAndSendEmail();
    }
    catch (Exception ^ep)
    {
        Console::WriteLine(ep->ToString());
    }

    Console::ReadKey();

    return 0;
}

TLS 1.2 Protocol

TLS is the successor of SSL, more and more SMTP servers require TLS 1.2 encryption now.

If your operating system is Windows XP/Vista/Windows 7/Windows 2003/2008/2008 R2/2012/2012 R2, you need to enable TLS 1.2 protocol in your operating system like this:

Enable TLS 1.2 on Windows XP/Vista/7/10/Windows 2008/2008 R2/2012

Appendix

Comments

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