ASP.NET

Google will find you many examples.

Quote:

Using Nuget MailKit/MimeKit, it doesn't register with .Net Core V2.2

var googleCredentials = await GoogleWebAuthorizationBroker.AuthorizeAsync(secrets, new[] { GmailService.Scope.MailGoogleCom }, email, CancellationToken.None);

var googleCredentials = await GoogleWebAuthorizationBroker.AuthorizeAsync(secrets, new[] { GmailService.Scope.MailGoogleCom, GmailService.Scope.GmailSend }, email, CancellationToken.None);

// Load the SMTP Credentials
using (var credentials = SendGmailAsync.Create_GoogleCredentials(ms))
{
    var result = await SendGmailAsync.Send_GmailAsync(await credentials, message, 5);
    SendGmailAsync.ProcessSendResult(model, result);
}

public static async Task<UserCredential> Create_GoogleCredentials(MAILSERVER ms)
{
    var secrets = new Google.Apis.Auth.OAuth2.ClientSecrets
    {
        ClientId = ms.OAuthCredentials.ClientId,
        ClientSecret = ms.OAuthCredentials.ClientSecret
    };

    var googleCredentials = await GoogleWebAuthorizationBroker.AuthorizeAsync(
        secrets,
        new[] { GmailService.Scope.MailGoogleCom, GmailService.Scope.GmailSend },
        ms.AccountName,
        CancellationToken.None
    );

    if (googleCredentials.Token.IsExpired(SystemClock.Default))
    {
        await googleCredentials.RefreshTokenAsync(CancellationToken.None);
    }

    return googleCredentials;

}

public static async Task<ServiceAccountCredential> Create_GoogleCredentialsWithAccessToken(MAILSERVER ms)
{
    var certBuffer = File.ReadAllBytes("projectName-xxxxxxxxxx.p12");
    var certificate = new X509Certificate2(certBuffer, "notasecret", X509KeyStorageFlags.Exportable);
    var serviceAccountEmail = "xxxxxxxxxx-xxxxxx.iam.gserviceaccount.com";
    var userAccountEmail = "xxx@xxxxxx.com";
    var credential = new ServiceAccountCredential(new ServiceAccountCredential.Initializer(serviceAccountEmail)
    {
        User = userAccountEmail,
        Scopes = new[] { GmailService.Scope.MailGoogleCom }
    }.FromCertificate(certificate));

    if (await credential.RequestAccessTokenAsync(CancellationToken.None))
    {
        var service = new GmailService(
            new Google.Apis.Services.BaseClientService.Initializer()
            {
                HttpClientInitializer = credential
            }
        );
    }

    return credential;
}

public static async Task<SendEmailCompletedEventArgs> Send_GmailServiceAsync(ServiceAccountCredential serviceCredentials, MimeMessage message, int retryCount)
{
    var currentTry = 0;
    while ((currentTry < retryCount))
    {
        try
        {
            using (var client = new SmtpClient())
            {
                client.Connect("smtp.gmail.com", 587, SecureSocketOptions.StartTls);
                client.AuthenticationMechanisms.Remove("XOAUTH2");
                var oauth2 = new SaslMechanismOAuth2(serviceCredentials.User, serviceCredentials.Token.AccessToken);
                client.Authenticate(oauth2);

                await client.SendAsync(message);
                client.Disconnect(true);
            }

            return new SendEmailCompletedEventArgs(null, false, null, currentTry);
        }
        catch (Exception ex)
        {
            currentTry++;
            if ((currentTry >= retryCount))
            {
                return new SendEmailCompletedEventArgs(ex, true, null, currentTry);
            }
        }

    }

    //  Code should never reach here, but without this line you'll get a BC42105 warning:
    return new SendEmailCompletedEventArgs(null, true, null, currentTry);
}

using (var credentials = SendGmailServiceP12Async.Create_GoogleCredentialsWithAccessToken(ms))
{
    var result = await SendGmailServiceP12Async.Send_GmailServiceAsync(await credentials, message, 5);
    SendGmailServiceP12Async.ProcessSendResult(model, result);
}

public static async Task<ServiceAccountCredential> Create_GoogleCredentialsWithAccessToken(MAILSERVER ms)
{
    ServiceAccountCredential credentials = null;<br />
    var serviceStream = await File.ReadAllTextAsync(ms.GoogleServiceFile);
    var parameters = new NewtonsoftJsonSerializer().Deserialize<GoogleServiceApi>(serviceStream);

    // Proceed as normal loading the credentials. 
    using (var stream = new FileStream(ms.GoogleServiceFile, FileMode.Open, FileAccess.Read))
    {
        ServiceAccountCredential original = (ServiceAccountCredential)
        GoogleCredential.FromStream(stream).UnderlyingCredential;

        var initializer = new ServiceAccountCredential.Initializer(original.Id)
        {
            User = parameters.user,
            ProjectId = parameters.project_id,
            Key = original.Key,
            Scopes = new[] { GmailService.Scope.MailGoogleCom, GmailService.Scope.GmailSend }
        };
        credentials = new ServiceAccountCredential(initializer);

        if (await credentials.RequestAccessTokenAsync(CancellationToken.None))
        {
            var service = new GmailService(
                new BaseClientService.Initializer()
                {
                    HttpClientInitializer = credentials
                }
            );
        }
    }

    return credentials;
}

public static async Task<SendEmailCompletedEventArgs> Send_GmailServiceAsync(ServiceAccountCredential serviceCredentials, MimeMessage message, int retryCount)
{
    var currentTry = 0;
    while ((currentTry < retryCount))
    {
        try
        {
            using (var client = new SmtpClient())
            {
                client.Connect("smtp.gmail.com", 587, SecureSocketOptions.StartTls);

                var oauth2 = new SaslMechanismOAuth2(serviceCredentials.User, serviceCredentials.Token.AccessToken);
                client.Authenticate(oauth2);

                await client.SendAsync(message);
                client.Disconnect(true);
            }

            return new SendEmailCompletedEventArgs(null, false, null, currentTry);
        }
        catch (Exception ex)
        {
            currentTry++;
            if ((currentTry >= retryCount))
            {
                return new SendEmailCompletedEventArgs(ex, true, null, currentTry);
            }
        }

    }

    //  Code should never reach here, but without this line you'll get a BC42105 warning:
    return new SendEmailCompletedEventArgs(null, true, null, currentTry);
}

public class GoogleServiceApi
{
    public string type { get; set; }
    public string user { get; set; }
    public string project_id { get; set; }
    public string private_key_id { get; set; }
    public string private_key { get; set; }
    public string client_email { get; set; }
    public string client_id { get; set; }
    public string auth_uri { get; set; }
    public string token_uri { get; set; }
    public string auth_provider_x509_cert_url { get; set; }
    public string client_x509_cert_url { get; set; }
}