gistfile1.txt

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

class Program
{
    static async Task Main()
    {

        using IHost host = Host.CreateDefaultBuilder()
            .ConfigureServices(services =>
            {
                services.AddSingleton<IDependency, Dependency>();
                // This line *is* required.
                services.AddSingleton<UnregisteredDependency>();
                services.AddHostedService<SampleHostedService>();
            })
            .Build();

        await host.RunAsync();
    }
}

public interface IDependency
{
    void LogPresence();
}

public class Dependency : IDependency
{
    public void LogPresence()
    {
        Console.WriteLine("Dependency presence");
    }
}

public class UnregisteredDependency
{
    private readonly IDependency _dependency;

    public UnregisteredDependency(IDependency dependency) =>
        _dependency = dependency;

    public void LogDependencyPresence() => _dependency.LogPresence();
}

public class SampleHostedService : IHostedService
{
    private readonly UnregisteredDependency _unregisteredDependency;

    public SampleHostedService(UnregisteredDependency unregisteredDependency) =>
        _unregisteredDependency = unregisteredDependency;

    public Task StartAsync(CancellationToken cancellationToken)
    {
        Console.WriteLine("Starting service");
        _unregisteredDependency.LogDependencyPresence();
        return Task.CompletedTask;
    }

    public Task StopAsync(CancellationToken cancellationToken)
    {
        return Task.CompletedTask;
    }
}