C#/ASP.NET/ASP MVC - 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
using System;
using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Security.Cryptography.X509Certificates;
using System.Runtime.InteropServices;
using Google.Apis.Auth.OAuth2;
using EAGetMail;
using System.Globalization;

// 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", new CultureInfo("en-US")),
        currentDateTime.Millisecond,
        sequence);
}

public static void RetrieveMailWithGSuiteUser()
{
    try
    {
        // service account email address
        const string serviceAccount = "xxxxxx@xxxxx.iam.gserviceaccount.com";

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

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

        var serviceAccountCredentialInitializer = new ServiceAccountCredential.Initializer(serviceAccount)
        {
            User = gsuiteUser,
            Scopes = new[] { "https://mail.google.com/" }

        }.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";

        // var serviceAccountCredentialInitializer = new ServiceAccountCredential.Initializer(serviceAccount)
        //{
        //    User = gsuiteUser,
        //    Scopes = new[] { "https://mail.google.com/" }
        // }.FromPrivateKey(privateKey);

        // request access token
        var credential = new ServiceAccountCredential(serviceAccountCredentialInitializer);
        if (!credential.RequestAccessTokenAsync(CancellationToken.None).Result)
            throw new 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);
        }

        MailServer oServer = new MailServer("imap.gmail.com",
                gsuiteUser,
                credential.Token.AccessToken, // use access token as password
                ServerProtocol.Imap4);

        // Set IMAP OAUTH 2.0
        oServer.AuthType = ServerAuthType.AuthXOAUTH2;
        // Enable SSL/TLS connection, most modern email server require SSL/TLS by default
        oServer.SSLConnection = true;
        // Set IMAP4 SSL Port
        oServer.Port = 993;

        // Since EAGetMail 5.3.5, Gmail Rest API is supported as well. You can use the following code
        // to retrieve email using Gmail Rest API.
        // MailServer oServer = new MailServer("gmail.googleapis.com",
        //        userEmail,
        //        accessToken, // use access token as password
        //        ServerProtocol.GmailRestApi);

        // oServer.AuthType = ServerAuthType.AuthXOAUTH2;
        // oServer.SSLConnection = true;

        MailClient oClient = new MailClient("TryIt");
        // Get new email only, if you want to get all emails, please remove this line
        oClient.GetMailInfosParam.GetMailInfosOptions = GetMailInfosOptionType.NewOnly;

        Console.WriteLine("Connecting {0} ...", oServer.Server);
        oClient.Connect(oServer);

        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);

            // Receive email from email server
            Mail oMail = oClient.GetMail(info);

            Console.WriteLine("From: {0}", oMail.From.ToString());
            Console.WriteLine("Subject: {0}\r\n", oMail.Subject);

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

            // 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 server.
        oClient.Quit();
        Console.WriteLine("Completed!");
    }
    catch (Exception ep)
    {
        Console.WriteLine(ep.ToString());
    }
}