Advanced - Background tasks with hosted services

Worker Service template

<Project Sdk="Microsoft.NET.Sdk.Worker">

Visual Studio
.NET CLI
Create a new project.
Select Worker Service. Select Next.
Provide a project name in the Project name field or accept the default project name. Select Next.
In the Additional information dialog, Choose a Framework. Select Create.

dotnet new worker -o ContosoWorker

Package

IHostedService interface

StartAsync(CancellationToken)
StopAsync(CancellationToken)

`StartAsync`

The app's request processing pipeline is configured.
The server is started and IApplicationLifetime.ApplicationStarted is triggered.

`StopAsync`

StopAsync(CancellationToken) is triggered when the host is performing a graceful shutdown. StopAsync contains the logic to end the background task. Implement IDisposable and finalizers (destructors) to dispose of any unmanaged resources.
Any remaining background operations that the app is performing should be aborted.
Any methods called in StopAsync should return promptly.
ShutdownTimeout when using Generic Host. For more information, see .NET Generic Host in ASP.NET Core.
Shutdown timeout host configuration setting when using Web Host. For more information, see ASP.NET Core Web Host.

BackgroundService base class

Timed background tasks

public class TimedHostedService : IHostedService, IDisposable
{
    private int executionCount = 0;
    private readonly ILogger<TimedHostedService> _logger;
    private Timer? _timer = null;

    public TimedHostedService(ILogger<TimedHostedService> logger)
    {
        _logger = logger;
    }

    public Task StartAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation("Timed Hosted Service running.");

        _timer = new Timer(DoWork, null, TimeSpan.Zero,
            TimeSpan.FromSeconds(5));

        return Task.CompletedTask;
    }

    private void DoWork(object? state)
    {
        var count = Interlocked.Increment(ref executionCount);

        _logger.LogInformation(
            "Timed Hosted Service is working. Count: {Count}", count);
    }

    public Task StopAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation("Timed Hosted Service is stopping.");

        _timer?.Change(Timeout.Infinite, 0);

        return Task.CompletedTask;
    }

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

services.AddHostedService<TimedHostedService>();

Consuming a scoped service in a background task

The service is asynchronous. The DoWork method returns a Task. For demonstration purposes, a delay of ten seconds is awaited in the DoWork method.
An ILogger is injected into the service.

internal interface IScopedProcessingService
{
    Task DoWork(CancellationToken stoppingToken);
}

internal class ScopedProcessingService : IScopedProcessingService
{
    private int executionCount = 0;
    private readonly ILogger _logger;
    
    public ScopedProcessingService(ILogger<ScopedProcessingService> logger)
    {
        _logger = logger;
    }

    public async Task DoWork(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            executionCount++;

            _logger.LogInformation(
                "Scoped Processing Service is working. Count: {Count}", executionCount);

            await Task.Delay(10000, stoppingToken);
        }
    }
}

public class ConsumeScopedServiceHostedService : BackgroundService
{
    private readonly ILogger<ConsumeScopedServiceHostedService> _logger;

    public ConsumeScopedServiceHostedService(IServiceProvider services, 
        ILogger<ConsumeScopedServiceHostedService> logger)
    {
        Services = services;
        _logger = logger;
    }

    public IServiceProvider Services { get; }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation(
            "Consume Scoped Service Hosted Service running.");

        await DoWork(stoppingToken);
    }

    private async Task DoWork(CancellationToken stoppingToken)
    {
        _logger.LogInformation(
            "Consume Scoped Service Hosted Service is working.");

        using (var scope = Services.CreateScope())
        {
            var scopedProcessingService = 
                scope.ServiceProvider
                    .GetRequiredService<IScopedProcessingService>();

            await scopedProcessingService.DoWork(stoppingToken);
        }
    }

    public override async Task StopAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation(
            "Consume Scoped Service Hosted Service is stopping.");

        await base.StopAsync(stoppingToken);
    }
}

services.AddHostedService<ConsumeScopedServiceHostedService>();
services.AddScoped<IScopedProcessingService, ScopedProcessingService>();

Queued background tasks

public interface IBackgroundTaskQueue
{
    ValueTask QueueBackgroundWorkItemAsync(Func<CancellationToken, ValueTask> workItem);

    ValueTask<Func<CancellationToken, ValueTask>> DequeueAsync(
        CancellationToken cancellationToken);
}

public class BackgroundTaskQueue : IBackgroundTaskQueue
{
    private readonly Channel<Func<CancellationToken, ValueTask>> _queue;

    public BackgroundTaskQueue(int capacity)
    {
        // Capacity should be set based on the expected application load and
        // number of concurrent threads accessing the queue.            
        // BoundedChannelFullMode.Wait will cause calls to WriteAsync() to return a task,
        // which completes only when space became available. This leads to backpressure,
        // in case too many publishers/calls start accumulating.
        var options = new BoundedChannelOptions(capacity)
        {
            FullMode = BoundedChannelFullMode.Wait
        };
        _queue = Channel.CreateBounded<Func<CancellationToken, ValueTask>>(options);
    }

    public async ValueTask QueueBackgroundWorkItemAsync(
        Func<CancellationToken, ValueTask> workItem)
    {
        if (workItem == null)
        {
            throw new ArgumentNullException(nameof(workItem));
        }

        await _queue.Writer.WriteAsync(workItem);
    }

    public async ValueTask<Func<CancellationToken, ValueTask>> DequeueAsync(
        CancellationToken cancellationToken)
    {
        var workItem = await _queue.Reader.ReadAsync(cancellationToken);

        return workItem;
    }
}

The BackgroundProcessing method returns a Task, which is awaited in ExecuteAsync.
Background tasks in the queue are dequeued and executed in BackgroundProcessing.
Work items are awaited before the service stops in StopAsync.

public class QueuedHostedService : BackgroundService
{
    private readonly ILogger<QueuedHostedService> _logger;

    public QueuedHostedService(IBackgroundTaskQueue taskQueue, 
        ILogger<QueuedHostedService> logger)
    {
        TaskQueue = taskQueue;
        _logger = logger;
    }

    public IBackgroundTaskQueue TaskQueue { get; }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation(
            $"Queued Hosted Service is running.{Environment.NewLine}" +
            $"{Environment.NewLine}Tap W to add a work item to the " +
            $"background queue.{Environment.NewLine}");

        await BackgroundProcessing(stoppingToken);
    }

    private async Task BackgroundProcessing(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            var workItem = 
                await TaskQueue.DequeueAsync(stoppingToken);

            try
            {
                await workItem(stoppingToken);
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, 
                    "Error occurred executing {WorkItem}.", nameof(workItem));
            }
        }
    }

    public override async Task StopAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation("Queued Hosted Service is stopping.");

        await base.StopAsync(stoppingToken);
    }
}

The IBackgroundTaskQueue is injected into the MonitorLoop service.
IBackgroundTaskQueue.QueueBackgroundWorkItem is called to enqueue a work item.
The work item simulates a long-running background task:
- Three 5-second delays are executed (Task.Delay).
- A try-catch statement traps OperationCanceledException if the task is cancelled.

public class MonitorLoop
{
    private readonly IBackgroundTaskQueue _taskQueue;
    private readonly ILogger _logger;
    private readonly CancellationToken _cancellationToken;

    public MonitorLoop(IBackgroundTaskQueue taskQueue,
        ILogger<MonitorLoop> logger,
        IHostApplicationLifetime applicationLifetime)
    {
        _taskQueue = taskQueue;
        _logger = logger;
        _cancellationToken = applicationLifetime.ApplicationStopping;
    }

    public void StartMonitorLoop()
    {
        _logger.LogInformation("MonitorAsync Loop is starting.");

        // Run a console user input loop in a background thread
        Task.Run(async () => await MonitorAsync());
    }

    private async ValueTask MonitorAsync()
    {
        while (!_cancellationToken.IsCancellationRequested)
        {
            var keyStroke = Console.ReadKey();

            if (keyStroke.Key == ConsoleKey.W)
            {
                // Enqueue a background work item
                await _taskQueue.QueueBackgroundWorkItemAsync(BuildWorkItem);
            }
        }
    }

    private async ValueTask BuildWorkItem(CancellationToken token)
    {
        // Simulate three 5-second tasks to complete
        // for each enqueued work item

        int delayLoop = 0;
        var guid = Guid.NewGuid().ToString();

        _logger.LogInformation("Queued Background Task {Guid} is starting.", guid);

        while (!token.IsCancellationRequested && delayLoop < 3)
        {
            try
            {
                await Task.Delay(TimeSpan.FromSeconds(5), token);
            }
            catch (OperationCanceledException)
            {
                // Prevent throwing if the Delay is cancelled
            }

            delayLoop++;

            _logger.LogInformation("Queued Background Task {Guid} is running. " 
                                   + "{DelayLoop}/3", guid, delayLoop);
        }

        if (delayLoop == 3)
        {
            _logger.LogInformation("Queued Background Task {Guid} is complete.", guid);
        }
        else
        {
            _logger.LogInformation("Queued Background Task {Guid} was cancelled.", guid);
        }
    }
}

services.AddSingleton<MonitorLoop>();
services.AddHostedService<QueuedHostedService>();
services.AddSingleton<IBackgroundTaskQueue>(ctx =>
{
    if (!int.TryParse(hostContext.Configuration["QueueCapacity"], out var queueCapacity))
        queueCapacity = 100;
    return new BackgroundTaskQueue(queueCapacity);
});

var monitorLoop = host.Services.GetRequiredService<MonitorLoop>();
monitorLoop.StartMonitorLoop();

Asynchronous timed background task

namespace TimedBackgroundTasks;

public class TimedHostedService : BackgroundService
{
    private readonly ILogger<TimedHostedService> _logger;
    private int _executionCount;

    public TimedHostedService(ILogger<TimedHostedService> logger)
    {
        _logger = logger;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation("Timed Hosted Service running.");

        // When the timer should have no due-time, then do the work once now.
        DoWork();

        using PeriodicTimer timer = new(TimeSpan.FromSeconds(1));

        try
        {
            while (await timer.WaitForNextTickAsync(stoppingToken))
            {
                DoWork();
            }
        }
        catch (OperationCanceledException)
        {
            _logger.LogInformation("Timed Hosted Service is stopping.");
        }
    }

    // Could also be a async method, that can be awaited in ExecuteAsync above
    private void DoWork()
    {
        int count = Interlocked.Increment(ref _executionCount);

        _logger.LogInformation("Timed Hosted Service is working. Count: {Count}", count);
    }
}

Native AOT

Visual Studio
.NET CLI
Create a new project.
Select Worker Service. Select Next.
Provide a project name in the Project name field or accept the default project name. Select Next.
In the Additional information dialog:
Choose a Framework.
Check the Enable Native AOT publish checkbox.
Select Create.

dotnet new worker -o WorkerWithAot --aot

<Project Sdk="Microsoft.NET.Sdk.Worker">

  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
    <InvariantGlobalization>true</InvariantGlobalization>
+   <PublishAot>true</PublishAot>
    <UserSecretsId>dotnet-WorkerWithAot-e94b2</UserSecretsId>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Extensions.Hosting" Version="8.0.0-preview.4.23259.5" />
  </ItemGroup>
</Project>

Additional resources

Background services unit tests on GitHub.
View or download sample code (how to download)
Implement background tasks in microservices with IHostedService and the BackgroundService class
Run background tasks with WebJobs in Azure App Service
Timer

Ref: Background tasks with hosted services in ASP.NET Core

Tags: Summary AspNetCore

Advanced - Background tasks with hosted services

In this article

Worker Service template

Package

IHostedService interface

StartAsync

StopAsync

BackgroundService base class

Timed background tasks

Consuming a scoped service in a background task

Queued background tasks

Asynchronous timed background task

Native AOT

Additional resources

`StartAsync`

`StopAsync`