Scheduler.Start();

public class Scheduler
{
    private readonly static Lazy<Scheduler> _instance = new Lazy<Scheduler>(() => new Scheduler());
    private static IScheduler scheduler;

    public static Scheduler Instance
    {
        get
        {
            return _instance.Value;
        }
    }

    public static async void Start()
    {
        scheduler = await StdSchedulerFactory.GetDefaultScheduler();
        await scheduler.Start();

        InicializarMonitoreo();

    }


    private static void InicializarMonitoreo()
    {
        EnviarCorreos();
    }

    public static void EnviarCorreos()
    {
        IJobDetail job = JobBuilder.Create<JobCorreoAdministradores>().WithIdentity("myJob", "myGroup").Build();

        ITrigger trigger = TriggerBuilder.Create()
            .WithSchedule(CronScheduleBuilder.CronSchedule("0 0/1 * 1/1 * ? *"))
            .Build();

        Scheduler.AddJob(job, trigger);
    }


    public static void AddJob(IJobDetail job, ITrigger trigger)
    {
        scheduler.ScheduleJob(job, trigger);
    }
}

public class JobCorreoAdministradores: IJob
{
    public Task Execute(IJobExecutionContext context)
    {
        List<string> emails = new List<string>();

        emails.Add("correo1@dominio.com");
        emails.Add("correo2@dominio.com");


        string asunto = "correo de prueba";
        string mensaje = "Hola este correo es de prueba para envio programado";

        EmailSender _emailSender = new EmailSender();
        _emailSender.SendMultipleRecipientsEmailAsync(emails, asunto, mensaje);

        return Task.CompletedTask;
    }
}

public class CorreoObjetosGeograficosAdministradores : IJob
{
    public Task Execute(IJobExecutionContext context)
    {
        List<string> emails = new List<string>();

        var usuarios = dbcontext.Tabla.where(usuario => usuario.usuariomodificacion != null).select(usuarios => usuarios.email).ToList();

        foreach (var item in usuarios)
        {
            emails.Add(item);
        }

        string asunto = "correo de prueba";
        string mensaje = "Hola este correo es de prueba para envio programado";

        EmailSender _emailSender = new EmailSender(UrlAplicaciones.CORREO_HOST, UrlAplicaciones.CORREO_PORT, UrlAplicaciones.CORREO_ENABLE_SSL, UrlAplicaciones.CORREO_USER_NAME, UrlAplicaciones.CORREO_PASSWORD);
        _emailSender.SendMultipleRecipientsEmailAsync(emails, asunto, mensaje);

        return Task.CompletedTask;
    }
}

public class MyJobFactory : IJobFactory
{
    private readonly IServiceProvider _serviceProvider;

    public MyJobFactory(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;
    }

    public IJob NewJob(TriggerFiredBundle bundle, IScheduler scheduler)
    {
        return (IJob)this._serviceProvider.GetService(bundle.JobDetail.JobType);
    }

    public void ReturnJob(IJob job)
    {
        var disposable = job as IDisposable;
        disposable?.Dispose();
    }
}

public class Scheduler
{
    private readonly static Lazy<Scheduler> _instance = new Lazy<Scheduler>(() => new Scheduler());
    private static IScheduler scheduler;

    public static Scheduler Instance
    {
        get
        {
            return _instance.Value;
        }
    }

    public static async void Start(IServiceProvider serviceProvider)
        {
            scheduler = await StdSchedulerFactory.GetDefaultScheduler();
            scheduler.JobFactory = new JobFactory(serviceProvider);
            await scheduler.Start();

            InicializarMonitoreo();

        }


    private static void InicializarMonitoreo()
    {
        EnviarCorreos();
    }

    public static void EnviarCorreos()
    {
        IJobDetail job = JobBuilder.Create<JobCorreoAdministradores>().WithIdentity("myJob", "myGroup").Build();

        ITrigger trigger = TriggerBuilder.Create()
            .WithSchedule(CronScheduleBuilder.CronSchedule("0 0/1 * 1/1 * ? *"))
            .Build();

        Scheduler.AddJob(job, trigger);
    }


    public static void AddJob(IJobDetail job, ITrigger trigger)
    {
        scheduler.ScheduleJob(job, trigger);
    }
}