Testing .NET Worker Services with WebApplicationFactory

The .NET Worker Services are ideal for background processing tasks. They use a .NET Generic Host, and you can easily configure dependency injection, logging, and configuration. However, setting up tests for them can be a bit tricky. The WebApplicationFactory<TEntryPoint> class, commonly used for ASP.NET Core integration tests, does not work with the generic host out of the box.

This article demonstrates how to set up a Worker Service, explains the problem encountered when using WebApplicationFactory, and shows the solution.

Worker Service Example

Below is a simple Worker Service that registers a background worker and a scoped service:

var builder = Host.CreateApplicationBuilder(args);
builder.Services.AddScoped<IFooService, FooService>();
builder.Services.AddHostedService<Worker>();

var host = builder.Build();
host.Run();


public class Worker(IServiceScopeFactory scopeFactory) : BackgroundService
{
    private readonly IServiceScopeFactory _scopeFactory = scopeFactory;

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            using var scope = _scopeFactory.CreateScope();
            var fooService = scope.ServiceProvider.GetRequiredService<IFooService>();
            fooService.Calculate();

            await Task.Delay(1000, stoppingToken);
        }
    }
}

public interface IFooService
{
    int Calculate();
}

public class FooService : IFooService
{
    public int Calculate()
    {
        return 11;
    }
}

The Testing Challenge

When you try to use WebApplicationFactory for a Worker Service, you may encounter this exception:

System.InvalidOperationException: No application configured. Please specify an application via IWebHostBuilder.UseStartup, IWebHostBuilder.Configure, or specifying the startup assembly via StartupAssemblyKey in the web host configuration.

This happens because Worker Services do not configure an HTTP request pipeline, which WebApplicationFactory expects by default.

The Solution: No-op Configure

To resolve this, add a no-op configuration in your test factory:

public class TestFactory : WebApplicationFactory<WorkerServiceMarker>
{
    protected override void ConfigureWebHost(IWebHostBuilder builder)
    {
        // This line is required to satisfy the host builder
        builder.Configure(_ => { });

        builder.ConfigureServices(services =>
        {
            // Optionally remove hosted services to prevent them from running during tests
            var hostedServiceDescriptors = services
                .Where(descriptor => descriptor.ServiceType == typeof(IHostedService))
                .ToList();

            foreach (var descriptor in hostedServiceDescriptors)
            {
                services.Remove(descriptor);
            }
        });
    }
}

The key line is builder.Configure(_ => { });. This provides a minimal, no-op application configuration, allowing the host to build successfully.

Summary

• Worker Services can be tested with WebApplicationFactory by adding a no-op Configure call.
• This enables you to use the full host, configuration, and DI setup in your tests.
• Optionally, you can remove hosted services during tests to prevent background tasks from running.

I hope you found this article useful. Happy coding!