How can I write unit test for my background service?

Question

I'm working with the HostBuilder in .NET Core (not the WebHost !).

I have one Hosted Service running in my application that overrides the ExecuteAsync/StopAsync methods of the background Service and I want to unit test it.

Here is my HostedService:

public class DeviceToCloudMessageHostedService : BackgroundService
{
    private readonly IDeviceToCloudMessageService _deviceToCloudMessageService;
    private readonly AppConfig _appConfig;

    public DeviceToCloudMessageHostedService(IDeviceToCloudMessageService deviceToCloudMessageService, IOptionsMonitor<AppConfig> appConfig)
    {
        _deviceToCloudMessageService = deviceToCloudMessageService;
        _appConfig = appConfig.CurrentValue;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            await _deviceToCloudMessageService.DoStuff(stoppingToken);
            await Task.Delay(_appConfig.Parameter1, stoppingToken);
        }
    }
    
    public override Task StopAsync(CancellationToken cancellationToken)
    {
        Log.Information("Task Cancelled");
        _deviceToCloudMessageService.EndStuff();
        return base.StopAsync(cancellationToken);
    }

I already found this post: Integration Test for Hosted Service in .NET Core

But it's explained for a QueuedBackgroundService and I don't really know if I can test mine the same way.

I just want to know if my code is executed. I don't want any specific result. Do you have any idea of how I can test it?

Answer 1

You should still be able to follow a similar format as the linked answer.

Mock the dependencies and inject them, invoke the methods under test and assert the expected behavior.

The following uses Moq to mock the dependencies along with ServiceCollection to do the heavy lifting of injecting the dependencies.

using System;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Moq;

[TestMethod]
public async Task DeviceToCloudMessageHostedService_Should_DoStuff() {
    //Arrange
    IServiceCollection services = new ServiceCollection();
    services.AddSingleton<IHostedService, DeviceToCloudMessageHostedService>();
    //mock the dependencies for injection
    services.AddSingleton(Mock.Of<IDeviceToCloudMessageService>(_ =>
        _.DoStuff(It.IsAny<CancellationToken>()) == Task.CompletedTask
    ));
    services.AddSingleton(Mock.Of<IOptionsMonitor<AppConfig>>(_ =>
        _.CurrentValue == Mock.Of<AppConfig>(c => 
            c.Parameter1 == TimeSpan.FromMilliseconds(1000)
        )
    ));
    var serviceProvider = services.BuildServiceProvider();
    var hostedService = serviceProvider.GetService<IHostedService>();

    //Act
    await hostedService.StartAsync(CancellationToken.None);
    await Task.Delay(1000);//Give some time to invoke the methods under test
    await hostedService.StopAsync(CancellationToken.None);

    //Assert
    var deviceToCloudMessageService = serviceProvider
        .GetRequiredService<IDeviceToCloudMessageService>();
    //extracting mock to do verifications
    var mock = Mock.Get(deviceToCloudMessageService);
    //assert expected behavior
    mock.Verify(_ => _.DoStuff(It.IsAny<CancellationToken>()), Times.AtLeastOnce);
    mock.Verify(_ => _.EndStuff(), Times.AtLeastOnce());
}

Now, ideally this would count as testing framework code since you are basically testing that a BackgroundService behaves as expected when run, but it should demonstrate enough about how one would test such a service in isolation