Managed C++/CLI - Retrieve email using Gmail/G Suite OAuth 2.0 + IMAP4 protocol in background service (service account)¶

// You can install Google.Apis.Auth.OAuth2 by NuGet
// Install-Package Google.Apis.Auth
#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 System::Security::Cryptography::X509Certificates;
using namespace System::Globalization;

using namespace Google::Apis::Auth::OAuth2;
using namespace EAGetMail;

// Generate an unqiue email file name based on date time
static String ^ _generateFileName(int sequence)
{
    DateTime currentDateTime = DateTime::Now;
    return String::Format("{0}-{1:000}-{2:000}.eml",
        currentDateTime.ToString("yyyyMMddHHmmss", gcnew CultureInfo("en-US")),
        currentDateTime.Millisecond,
        sequence);
}

static void RetrieveWithGSuiteUser()
{
    try
    {
        // service account email address
        String^ serviceAccount = "xxxxxx@xxxxx.iam.gserviceaccount.com";

        // import service account key p12 certificate.
        auto certificate = gcnew X509Certificate2("D:\\MyData\\myoauth-77dec4d192ec.p12",
            "notasecret", X509KeyStorageFlags::Exportable);

        // G Suite user email address
        String^ gsuiteUser = "user@gsuitdomain.com";

        auto serviceAccountCredentialInitializer = gcnew ServiceAccountCredential::Initializer(serviceAccount);

        {
            serviceAccountCredentialInitializer->User = gsuiteUser;
            serviceAccountCredentialInitializer->Scopes = gcnew array<String^>{ "https://mail.google.com/" };
        }

        serviceAccountCredentialInitializer->FromCertificate(certificate);

        // if service account key is in json format, copy the private key from json file:
        // "private_key": "-----BEGIN PRIVATE KEY-----\n...\n-----END PRIVATE KEY-----\n"
        // and import it like this:
        // String^ privateKey = "-----BEGIN PRIVATE KEY-----\nMIIEv...revdd\n-----END PRIVATE KEY-----\n";

        // serviceAccountCredentialInitializer->FromPrivateKey(privateKey);

        // request access token
        auto credential = gcnew ServiceAccountCredential(serviceAccountCredentialInitializer);
        if (!credential->RequestAccessTokenAsync(CancellationToken::None)->Result)
            throw gcnew InvalidOperationException("Access token failed.");


        // Create a folder named "inbox" under current directory
        // to save the email retrieved.
        String ^localInbox = String::Format("{0}\\inbox", Directory::GetCurrentDirectory());

        // If the folder is not existed, create it.
        if (!Directory::Exists(localInbox))
        {
            Directory::CreateDirectory(localInbox);
        }

        // Gmail IMAP4 Server
        MailServer ^oServer = gcnew MailServer("imap.gmail.com",
            gsuiteUser,
            credential->Token->AccessToken, // use access token as password
            ServerProtocol::Imap4);

        // Use IMAP OAUTH 2.0
        oServer->AuthType = ServerAuthType::AuthXOAUTH2;
        // Enable SSL/TLS connection
        oServer->SSLConnection = true;
        // Set IMAP4 SSL port
        oServer->Port = 993;

        Console::WriteLine("Connecting {0}", oServer->Server);

        MailClient ^oClient = gcnew MailClient("TryIt");
        oClient->Connect(oServer);

        // Get new email only, if you want to get all emails, please remove this line
        oClient->GetMailInfosParam->GetMailInfosOptions = GetMailInfosOptionType::NewOnly;

        array<MailInfo^> ^infos = oClient->GetMailInfos();
        Console::WriteLine("Total {0} email(s)\r\n", infos->Length);

        for (int i = 0; i < infos->Length; i++)
        {
            MailInfo ^info = infos[i];
            Console::WriteLine("Index: {0}; Size: {1}; UIDL: {2}",
                info->Index, info->Size, info->UIDL);

            // Generate an unqiue email file name based on date time
            String^ fileName = _generateFileName(i + 1);
            String^ fullPath = String::Format("{0}\\{1}", localInbox, fileName);

            // Receive email from IMAP server
            EAGetMail::Mail ^oMail = oClient->GetMail(info);
            Console::WriteLine("From: {0}", oMail->From->ToString());
            Console::WriteLine("Subject: {0}\r\n", oMail->Subject);

            // Save email to local disk
            oMail->SaveAs(fullPath, true);

            // Mark email as read to prevent retrieving this email again.
            oClient->MarkAsRead(info, true);

            // If you want to delete current email, please use Delete method instead of MarkAsRead
            // oClient->Delete(info);
        }

        // Quit and expunge emails marked as deleted from IMAP server.
        oClient->Quit();
        Console::WriteLine("Completed!");

    }
    catch (Exception^ ep)
    {
        Console::WriteLine(ep->ToString());
    }
}