schedule cron job. IHostedService, asp.net core

CronJobService.cs

public abstract class CronJobService : IHostedService, IDisposable
{
    private System.Timers.Timer _timer;
    private readonly CronExpression _expression;
    private readonly TimeZoneInfo _timeZoneInfo;

    protected CronJobService(string cronExpression, TimeZoneInfo timeZoneInfo)
    {
        _expression = CronExpression.Parse(cronExpression);
        _timeZoneInfo = timeZoneInfo;
    }

    public virtual async Task StartAsync(CancellationToken cancellationToken)
    {
        await ScheduleJob(cancellationToken);
    }

    protected virtual async Task ScheduleJob(CancellationToken cancellationToken)
    {
        var next = _expression.GetNextOccurrence(DateTimeOffset.Now, _timeZoneInfo);
        if (next.HasValue)
        {
            var delay = next.Value - DateTimeOffset.Now;

            // interval can be int.MaxValue at max
            if (Math.Ceiling(delay.TotalMilliseconds) > int.MaxValue)
            {
                // set timer to int.MaxValue
                _timer = new System.Timers.Timer(int.MaxValue);
                _timer.Elapsed += async (sender, args) =>
                {
                    _timer.Dispose();  // reset and dispose timer
                    _timer = null;

                    // no work to be done here, this is in between desired executions
                    //if (!cancellationToken.IsCancellationRequested)
                    //{
                    //    await DoWork(cancellationToken);
                    //}

                    if (!cancellationToken.IsCancellationRequested)
                    {
                        await ScheduleJob(cancellationToken);    // reschedule next
                    }
                };
                _timer.Start();
            }
            else
            {
                _timer = new System.Timers.Timer(delay.TotalMilliseconds);
                _timer.Elapsed += async (sender, args) =>
                {
                    _timer.Dispose();  // reset and dispose timer
                    _timer = null;

                    if (!cancellationToken.IsCancellationRequested)
                    {
                        await DoWork(cancellationToken);
                    }

                    if (!cancellationToken.IsCancellationRequested)
                    {
                        await ScheduleJob(cancellationToken);    // reschedule next
                    }
                };
                _timer.Start();
            }
        }
        await Task.CompletedTask;
    }

    public virtual async Task DoWork(CancellationToken cancellationToken)
    {
        await Task.Delay(5000, cancellationToken);  // do the work
    }

    public virtual async Task StopAsync(CancellationToken cancellationToken)
    {
        _timer?.Stop();
        await Task.CompletedTask;
    }

    public virtual void Dispose()
    {
        _timer?.Dispose();
    }
}