All 15 Background Services questions
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What is the difference between IHostedService and BackgroundService, and when would you implement IHostedService directly? How does graceful shutdown work for a BackgroundService, and what role does CancellationToken play? How should exceptions be handled inside a BackgroundService's ExecuteAsync loop to prevent the entire service from crashing silently? How does a BackgroundService differ from a Worker Service project template, and how do they relate? How do you register and run multiple independent BackgroundServices within the same application? How would you implement a scheduled (cron-like) background job in .NET without a third-party library? What is the significance of the order of AddHostedService registrations relative to StartAsync execution, and can you control it? How should a BackgroundService handle a fatal startup dependency failure, such as being unable to connect to a required message queue at launch? How do you safely access scoped services (like a DbContext) from within a BackgroundService, which itself is registered as a singleton? What is IHostedLifecycleService, and how does it provide more granular hooks than IHostedService's StartAsync/StopAsync?
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.NET › Background Services

What is a BackgroundService in ASP.NET Core, and what does it provide out of the box?

Beginner

Answer

BackgroundService is an abstract base class implementing IHostedService, designed for long-running background work that runs alongside the main application (like an ASP.NET Core web app or worker service). You override its ExecuteAsync(CancellationToken) method with your work loop, and the host manages starting it at application startup and gracefully signaling cancellation at shutdown.
public class EmailQueueProcessor : BackgroundService {
    protected override async Task ExecuteAsync(CancellationToken stoppingToken) {
        while (!stoppingToken.IsCancellationRequested) {
            await ProcessQueueAsync();
            await Task.Delay(TimeSpan.FromSeconds(5), stoppingToken);
        }
    }
}

// Registration
builder.Services.AddHostedService<EmailQueueProcessor>();
Real-world example A web API registers a BackgroundService that polls a message queue every few seconds and processes pending email notifications, running continuously in the same process as the API without needing a separate deployed service.

Common follow-ups: What's the difference between BackgroundService and implementing IHostedService directly?;How does the host handle exceptions thrown inside ExecuteAsync?

Generic Host;Worker Services & IHostedService
All 15 Background Services questions