Background tasks with hosted services in ASP.NET Core

By Jeow Li Huan

In ASP.NET Core, background tasks can be implemented as hosted services. A hosted service is a class with background task logic that implements the IHostedService interface. This article provides three hosted service examples:

• Background task that runs on a timer.
• Hosted service that activates a scoped service. The scoped service can use dependency injection (DI).
• Queued background tasks that run sequentially.

Worker Service template

The ASP.NET Core Worker Service template provides a starting point for writing long running service apps. An app created from the Worker Service template specifies the Worker SDK in its project file:

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

To use the template as a basis for a hosted services app:

Visual Studio

1. Create a new project.
2. Select Worker Service. Select Next.
3. Provide a project name in the Project name field or accept the default project name. Select Next.
4. In the Additional information dialog, Choose a Framework. Select Create.

.NET CLI

Use the Worker Service (worker) template with the dotnet new command from a command shell. In the following example, a Worker Service app is created named ContosoWorker. A folder for the ContosoWorker app is created automatically when the command is executed.

dotnet new worker -o ContosoWorker

Package

An app based on the Worker Service template uses the Microsoft.NET.Sdk.Worker SDK and has an explicit package reference to the Microsoft.Extensions.Hosting package. For example, see the sample app's project file (BackgroundTasksSample.csproj).

For web apps that use the Microsoft.NET.Sdk.Web SDK, the Microsoft.Extensions.Hosting package is referenced implicitly from the shared framework. An explicit package reference in the app's project file isn't required.

IHostedService interface

The IHostedService interface defines two methods for objects that are managed by the host:

• StartAsync(CancellationToken)
• StopAsync(CancellationToken)

StartAsync

StartAsync(CancellationToken) contains the logic to start the background task. StartAsync is called before:

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

StartAsync should be limited to short running tasks because hosted services are run sequentially, and no further services are started until StartAsync runs to completion.

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.

The cancellation token has a default 30 second timeout to indicate that the shutdown process should no longer be graceful. When cancellation is requested on the token:

• Any remaining background operations that the app is performing should be aborted.
• Any methods called in StopAsync should return promptly.

However, tasks aren't abandoned after cancellation is requested—the caller awaits all tasks to complete.

If the app shuts down unexpectedly (for example, the app's process fails), StopAsync might not be called. Therefore, any methods called or operations conducted in StopAsync might not occur.

To extend the default 30 second shutdown timeout, set:

• 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.

The hosted service is activated once at app startup and gracefully shut down at app shutdown. If an error is thrown during background task execution, Dispose should be called even if StopAsync isn't called.

BackgroundService base class

BackgroundService is a base class for implementing a long running IHostedService.

ExecuteAsync(CancellationToken) is called to run the background service. The implementation returns a Task that represents the entire lifetime of the background service. No further services are started until ExecuteAsync becomes asynchronous, such as by calling await. Avoid performing long, blocking initialization work in ExecuteAsync. The host blocks in StopAsync(CancellationToken) waiting for ExecuteAsync to complete.

The cancellation token is triggered when IHostedService.StopAsync is called. Your implementation of ExecuteAsync should finish promptly when the cancellation token is fired in order to gracefully shut down the service. Otherwise, the service ungracefully shuts down at the shutdown timeout. For more information, see the IHostedService interface section.

For more information, see the BackgroundService source code.

Timed background tasks

A timed background task makes use of the System.Threading.Timer class. The timer triggers the task's DoWork method. The timer is disabled on StopAsync and disposed when the service container is disposed on Dispose:

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();
    }
}

The Timer doesn't wait for previous executions of DoWork to finish, so the approach shown might not be suitable for every scenario. Interlocked.Increment is used to increment the execution counter as an atomic operation, which ensures that multiple threads don't update executionCount concurrently.

The service is registered in IHostBuilder.ConfigureServices (Program.cs) with the AddHostedService extension method:

services.AddHostedService<TimedHostedService>();

Consuming a scoped service in a background task

To use scoped services within a BackgroundService, create a scope. No scope is created for a hosted service by default.

The scoped background task service contains the background task's logic. In the following example:

• 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);
        }
    }
}

The hosted service creates a scope to resolve the scoped background task service to call its DoWork method. DoWork returns a Task, which is awaited in ExecuteAsync:

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);
    }
}

The services are registered in IHostBuilder.ConfigureServices (Program.cs). The hosted service is registered with the AddHostedService extension method:

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

Queued background tasks

A background task queue is based on the .NET 4.x QueueBackgroundWorkItem:

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;
    }
}

In the following QueueHostedService example:

• 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);
    }
}

A MonitorLoop service handles enqueuing tasks for the hosted service whenever the w key is selected on an input device:

• 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);
        }
    }
}

The services are registered in IHostBuilder.ConfigureServices (Program.cs). The hosted service is registered with the AddHostedService extension method:

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);
});

MonitorLoop is started in Program.cs:

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

Asynchronous timed background task

The following code creates an 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

The Worker Service templates support .NET native ahead-of-time (AOT) with the --aot flag:

Visual Studio

1. Create a new project.
2. Select Worker Service. Select Next.
3. Provide a project name in the Project name field or accept the default project name. Select Next.
4. In the Additional information dialog:
5. Choose a Framework.
6. Check the Enable Native AOT publish checkbox.
7. Select Create.

.NET CLI

Use the Worker Service (worker) template with the dotnet new command from a command shell with the AOT option:

dotnet new worker -o WorkerWithAot --aot

The AOT option adds <PublishAot>true</PublishAot> to the project file:

<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