What is IHostedService?

It is an interface used for implementing background tasks that run with the application.

IHost vs IHostBuilder vs IHostedService in .NET Core | FullStackPrep.Dev

Q: What is IHostBuilder used for?

It is used to configure services and create an IHost for running the app.

💡 Concept Name

IHost, IHostBuilder, and IHostedService

📘 Quick Intro

These interfaces are part of the Generic Host in .NET Core and help structure the startup, configuration, and background processing lifecycle of an application.

🧠 Analogy / Short Story

Imagine building a factory:

IHostBuilder is like the construction blueprint.
IHost is the built and running factory.
IHostedService is the machine that runs in the factory doing background work.

🔧 Technical Explanation

IHostBuilder: Used to configure services, logging, and app config. Creates an IHost object.
IHost: Manages app lifetime, DI container, and hosted services.
IHostedService: Interface for running background tasks. Registered in DI and managed by the host.

🎯 Purpose & Use Case

✅ Use IHostBuilder in Program.cs to bootstrap the app
✅ Use IHost to run and manage the app lifetime
✅ Use IHostedService for long-running background processes like worker queues

💻 Real Code Example

// Program.cs
var host = Host.CreateDefaultBuilder(args)
    .ConfigureServices(services =>
    {
        services.AddHostedService();
    })
    .Build();

await host.RunAsync();

// IHostedService implementation
public class MyWorker : IHostedService
{
    public Task StartAsync(CancellationToken cancellationToken)
    {
        Console.WriteLine("Background task started");
        return Task.CompletedTask;
    }

    public Task StopAsync(CancellationToken cancellationToken)
    {
        Console.WriteLine("Background task stopped");
        return Task.CompletedTask;
    }
}

❓ Interview Q&A

Q1: What is IHostBuilder used for?
A: To configure and build the application host.

Q2: What does IHost do?
A: Runs the app, manages services, and application lifetime.

Q3: Why use IHostedService?
A: To implement background tasks and services.

Q4: Can I use multiple IHostedService classes?
A: Yes, you can register multiple hosted services.

Q5: Where is IHost created?
A: From IHostBuilder using Build().

Q6: Can IHostBuilder configure DI?
A: Yes, via ConfigureServices.

Q7: Which method starts the host?
A: Run() or RunAsync().

Q8: Is IHostedService started automatically?
A: Yes, on application start.

Q9: What interface extends IHostedService for timers?
A: BackgroundService.

Q10: Which namespace includes these interfaces?
A: Microsoft.Extensions.Hosting.

📝 MCQs

Q1. What is the role of IHostBuilder?

Run the app
Host HTTP server
Configure and build the app host
Manage threads

Q2. What interface runs the application?

IServiceCollection
IConfiguration
IHost
IWebHost

Q3. What interface is used for background tasks?

IBackgroundTask
IWorker
IHostedService
ITaskService

Q4. Where do you register an IHostedService?

In Program.cs comments
In Startup.cs Main
In ConfigureServices
In appsettings.json

Q5. Which method is used to start IHost?

Execute()
RunAsync()
Start()
Build()

Q6. Which interface provides lifecycle hooks for background services?

ILogger
IOptions
IHostedService
IApplicationLifetime

Q7. Can you use multiple IHostedServices?

No
Only one allowed
Yes
Depends on OS

Q8. Which namespace contains IHost/IHostedService?

System.Threading
Microsoft.AspNetCore.Hosting
Microsoft.Extensions.Hosting
System.ServiceModel

Q9. What does IHostBuilder return?

ILoggerFactory
HttpClient
IWebApplication
An IHost instance

Q10. What base class simplifies IHostedService?

BaseWorker
TaskService
ServiceBase
BackgroundService

💡 Bonus Insight

The BackgroundService class is a built-in abstract implementation of IHostedService, ideal for long-running hosted tasks using ExecuteAsync.

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IHost vs IHostBuilder vs IHostedService in .NET Core