Handling application lifetime events for a hosted service

The IApplicationLifetime interface allows hosted services to gracefully handle startup and shutdown events. It has three properties of type CancellationToken.

• ApplicationStarted
• ApplicationStopping
• ApplicationStopped

You can register callbacks for one or more of those notifications.

public HostedService(
  ILogger<HostedService> logger, 
  IApplicationLifetime applicationLifetime)
{
    _logger = logger;
    _applicationLifetime = applicationLifetime;
}

public Task StartAsync(CancellationToken cancellationToken)
{
    _logger.LogInformation("Entering StartAsync");

    _applicationLifetime.ApplicationStarted.Register(OnStarted);
    _applicationLifetime.ApplicationStopping.Register(OnStopping);
    _applicationLifetime.ApplicationStopped.Register(OnStopped);

    _timer = new Timer(
        ScheduledTask,
        this,
        TimeSpan.FromSeconds(3),
        TimeSpan.FromSeconds(3));

    return Task.CompletedTask;
}

public Task StopAsync(CancellationToken cancellationToken)
{
    _logger.LogInformation("Entering StopAsync");

    return Task.CompletedTask;
}

private void OnStarted()
{
    _logger.LogInformation("OnStarted called.");
}

private void OnStopping()
{
    _logger.LogInformation("OnStopping called.");
}

private void OnStopped()
{
    _logger.LogInformation("OnStopped called.");
}

The IApplicationLifetime interface also has a StopApplication method which unsurprisingly does just that, which is to terminate the current application.

private void ScheduledTask(object state)
{
    var source = state as HostedService;

    _logger.LogInformation($"ScheduledTask invocation {source._count} via timer.");

    if (source._count++ == 3)
    {
        _logger.LogInformation("Shutting down...");
        source._applicationLifetime.StopApplication();
    };
}

The screenshot below shows the order in which these notifications are fired. It’s quite intuitive and just as you’d expect.

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