# Background tasks with hosted services in ASP.NET Core
By [Luke Latham](https://github.com/guardrex)
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](/dotnet/api/microsoft.extensions.hosting.ihostedservice) interface. This topic 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.
[View or download sample code](https://github.com/aspnet/Docs/tree/master/aspnetcore/fundamentals/host/hosted-services/samples/) ([how to download](xref:tutorials/index#how-to-download-a-sample))
* Web Host – The Web Host is useful for hosting web apps. The example code shown in this topic is from the Web Host version of the sample. For more information, see the [Web Host](xref:fundamentals/host/web-host) topic.
* Generic Host – The Generic Host is new in ASP.NET Core 2.1. For more information, see the [Generic Host](xref:fundamentals/host/generic-host) topic.
Hosted services implement the [IHostedService](/dotnet/api/microsoft.extensions.hosting.ihostedservice) interface. The interface defines two methods for objects that are managed by the host:
* [StartAsync(CancellationToken)](/dotnet/api/microsoft.extensions.hosting.ihostedservice.startasync) - `StartAsync` contains the logic to start the background task. When using the [Web Host](xref:fundamentals/host/web-host), `StartAsync` is called after the server has started and [IApplicationLifetime.ApplicationStarted](/dotnet/api/microsoft.aspnetcore.hosting.iapplicationlifetime.applicationstarted) is triggered. When using the [Generic Host](xref:fundamentals/host/generic-host), `StartAsync` is called before `ApplicationStarted` is triggered.
* [StopAsync(CancellationToken)](/dotnet/api/microsoft.extensions.hosting.ihostedservice.stopasync) - Triggered when the host is performing a graceful shutdown. `StopAsync` contains the logic to end the background task and dispose of any unmanaged resources. If the app shuts down unexpectedly (for example, the app's process fails), `StopAsync` might not be called.
The hosted service is activated once at app startup and gracefully shutdown at app shutdown. When [IDisposable](/dotnet/api/system.idisposable) is implemented, resources can be disposed when the service container is disposed. If an error is thrown during background task execution, `Dispose` should be called even if `StopAsync` isn't called.
A timed background task makes use of the [System.Threading.Timer](/dotnet/api/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`:
## Consuming a scoped service in a background task
To use scoped services within an `IHostedService`, 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, [ILogger](/dotnet/api/microsoft.extensions.logging.ilogger) is injected into the service:
The services are registered in `Startup.ConfigureServices`. The `IHostedService` implementation is registered with the `AddHostedService` extension method:
A background task queue is based on the .NET 4.x [QueueBackgroundWorkItem](/dotnet/api/system.web.hosting.hostingenvironment.queuebackgroundworkitem) ([tentatively scheduled to be built-in for ASP.NET Core 3.0](https://github.com/aspnet/Hosting/issues/1280)):
In `QueueHostedService`, background tasks in the queue are dequeued and executed as a [BackgroundService](/dotnet/api/microsoft.extensions.hosting.backgroundservice), which is a base class for implementing a long running `IHostedService`:
The services are registered in `Startup.ConfigureServices`. The `IHostedService` implementation is registered with the `AddHostedService` extension method:
When the **Add Task** button is selected on the Index page, the `OnPostAddTask` method is executed. `QueueBackgroundWorkItem` is called to enqueue the work item:
* [Implement background tasks in microservices with IHostedService and the BackgroundService class](/dotnet/standard/microservices-architecture/multi-container-microservice-net-applications/background-tasks-with-ihostedservice)
