Injecting Simple Injector components into IHostedService with ASP.NET Core 2.0

There are multiple ways to approach this. The simplest way is probably to cross-wire the hosted service in such way that the built-in configuration system resolves the hosted service from Simple Injector:

// Register in Simple Injector as Singleton
container.RegisterSingleton<THostedService>();

// Cross-wire TimedService in the built-in configuration system
services.AddSingleton<IHostedService>(
    c => container.GetInstance<TimedService>());

Do note that hosted services are resolved just once, and cached for ever, effectively making them Singletons. This is why you should register it in Simple Injector as Singleton.

Concequence of this, however, is that you won't be able to inject any Scoped or Transient dependencies into your hosted service. On top of that, it forces you to let your application component (TimedService) have a dependency on an ASP.NET Core abstraction (IHostedService). This is not ideal.

My preferred approach, therefore, is to instead create an adapter implementation that you register with the ASP.NET Core configuration system that forwards the calls to Simple Injector while using an application-specific abstraction to implement your service. So instead creating many IHostedService implementations, you define an abstraction that is specific and ideal to your application. Let's call this abstraction IMyJob.

The IHostedService adapter implementation might look like this:

public class SimpleInjectorJobProcessorHostedService : IHostedService, IDisposable
{
    private readonly Container container;
    private Timer timer;

    public SimpleInjectorJobProcessorHostedService(Container c) => this.container = c;

    public Task StartAsync(CancellationToken cancellationToken)
    {
        this.timer = new Timer(this.DoWork, null, TimeSpan.Zero, TimeSpan.FromSeconds(5));
        return Task.CompletedTask;
    }

    private void DoWork(object state)
    {
        // Run operation in a scope
        using (AsyncScopedLifestyle.BeginScope(this.container))
        {
            // Resolve the collection of IMyJob implementations
            foreach (var service in this.container.GetAllInstances<IMyJob>())
            {
                service.DoWork();
            }
        }
    }

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

    public void Dispose() => this.timer?.Dispose();
}

You can register it in ASP.NET core as follows:

services.AddSingleton<IHostedService>(
    new SimpleInjectorJobProcessorHostedService(container)); 

This way the actual jobs that you run can stay oblivious to ASP.NET Core and can be defined as follows:

public class CoolJob : IMyJob
{
    private readonly IJobRepository repo;

    public CoolJob(IJobRepository repo) => this.repo = repo;

    public void DoWork() => ...
}

And all jobs can be registered in Simple Injector as follows:

 // NOTE: Simple Injector v4.3 API
container.Collection.Register<IMyJob>(typeof(CoolJob).Assembly);

I would hook onto the ConfigureContainer method of the HostBuilder and setup simpleinjectore there like this:

                   IHostBuilder()
                   .ConfigureContainer<ServiceCollection>((builder, services) =>
                   {
                       var container = new Container();

                       container.RegisterSingleton<IJobRepository, JobRepository>();
                       services.AddTransient<IHostedService, TimedService>();

                   })
                   .ConfigureServices((hostContext, services) =>
                   {
                       // Originally we would have done this
                       //services.AddHostedService<Service>();
                   })
                   .Build();

        using (host)
        {
            await host.StartAsync();
            await host.WaitForShutdownAsync();
        }

While you could use your IHostedService implementation indeed I think it may hide what is going on. I believe the infrastructure bootstrapping should be done in one place or orchestrated at least in one place. I consider the container to be infrastructure and would set it all up with the rest of the app via the HostBuilder methods.

An added advantage may also be that you do not entirely replace the ServiceCollection as it works well with doing other framework related things. An example of some stuff I would still do with the ServiceCollection:

                   IHostBuilder()
                   .ConfigureServices((hostContext, services) =>
                   {
                       services.AddLogging();
                       services.AddOptions();
                   })

This is in line with what is stated in the simpleinjector docs about setting the container up with ASP.NET Core:

The practice with Simple Injector is to use Simple Injector to build up object graphs of your application components and let the built-in container build framework and third-party components,The practice with Simple Injector is to use Simple Injector to build up object graphs of your application components and let the built-in container build framework and third-party components

The same should apply with just .net core and the generic HostBuilder.