The ASP.NET Core docs for background services show a number of implementation examples.

There's an example for starting a service on a timer, though it's synchronous. There's another example which is asynchronous, for starting a service with a scoped dependency.

I need to do both: start a service every 5 minutes, and it has scoped dependencies. There's no example for that.

I combined both examples, but I'm unsure of a safe way to use Timer with an async TimerCallback.

e.g.

public class MyScheduler : IHostedService
{
  private Timer? _timer;
  private readonly IServiceScopeFactory _serviceScopeFactory;

  public MyScheduler(IServiceScopeFactory serviceScopeFactory) => _serviceScopeFactory = serviceScopeFactory;

  public void Dispose() => _timer?.Dispose();

  public Task StartAsync(CancellationToken cancellationToken)
  {
    _timer = new Timer((object? state) => {
      using var scope = _serviceScopeFactory.CreateScope();
      var myService = scope.ServiceProvider.GetRequiredService<IMyService>();
      await myService.Execute(cancellationToken);            // <------ problem
    }), null, TimeSpan.Zero, TimeSpan.FromSeconds(5));

    return Task.CompletedTask;
  }

  public Task StopAsync(CancellationToken cancellationToken) {
    _timer?.Change(Timeout.Infinite, 0);
    return Task.CompletedTask;
  }

}

The timer takes a sync callback, so the problem is the await. What's a safe way to call an async service?

CodePudding user response：

Use BackgroundService instead of IHostedService

public class MyScheduler : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        // Option 1
        while (!stoppingToken.IsCancellationRequested)
        {
            // do async work
            ...
            await Task.Delay(TimeSpan.FromMinutes(5), stoppingToken);
        }

        // Option 2 (.NET 6)
        var timer = new PeriodicTimer(TimeSpan.FromMinutes(5));

        while (await timer.WaitForNextTickAsync(stoppingToken))
        {
            // do async work
        }
    }
}

CodePudding user response：

Create an event with async handler and raise it every interval. The event handler can be awaited

public class MyScheduler : IHostedService {
    private Timer? _timer;
    private readonly IServiceScopeFactory _serviceScopeFactory;

    public MyScheduler(IServiceScopeFactory serviceScopeFactory) => _serviceScopeFactory = serviceScopeFactory;

    public void Dispose() => _timer?.Dispose();
    
    public Task StopAsync(CancellationToken cancellationToken) {
        _timer?.Change(Timeout.Infinite, 0);
        return Task.CompletedTask;
    }
    
    public Task StartAsync(CancellationToken cancellationToken) {
        performAction  = onPerformAction; //subscribe to event
        ScopedServiceArgs args = new ScopedServiceArgs {
            ServiceScopeFactory = _serviceScopeFactory,
            CancellationToken = cancellationToken
        };
        _timer = new Timer((object? state) =>  performAction(this, args), //<-- raise event
            null, TimeSpan.Zero, TimeSpan.FromSeconds(5));
        return Task.CompletedTask;
    }
  
    private event EventHandler<ScopedServiceArgs> performAction = delegate { };
    
    private async void onPerformAction(object sender, CancellationArgs args) {
        using IServiceScope scope = args.ServiceScopeFactory.CreateScope();
        IMyService myService = scope.ServiceProvider.GetRequiredService<IMyService>();
        await myService.Execute(args.CancellationToken);
    }
    
    class ScopedServiceArgs : EventArgs {
        public IServiceScopeFactory ServiceScopeFactory {get; set;}
        public CancellationToken CancellationToken {get; set;}
    }

}