<ItemGroup>
  <PackageReference Include="Microsoft.Extensions.Hosting" Version="9.0.*" />
</ItemGroup>

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

var host = Host.CreateDefaultBuilder(args)
    .ConfigureServices((context, services) =>
    {
        services.AddSingleton<IMyService, MyService>();
        services.AddSingleton<App>();
    })
    .Build();

var app = host.Services.GetRequiredService<App>();
await app.RunAsync();

namespace MyApp;

public sealed class App(IMyService myService)
{
    private readonly IMyService _myService = myService;

    public async Task RunAsync()
    {
        await _myService.DoWorkAsync();
    }
}

var host = Host.CreateDefaultBuilder(args)
    .ConfigureAppConfiguration((context, config) =>
    {
        config.AddJsonFile("appsettings.json", optional: true);
    })
    .ConfigureServices((context, services) =>
    {
        services.Configure<AppSettings>(
            context.Configuration.GetSection("AppSettings"));
        services.AddSingleton<App>();
    })
    .Build();

public sealed class App(ILogger<App> logger)
{
    public Task RunAsync()
    {
        logger.LogInformation("애플리케이션 시작");
        return Task.CompletedTask;
    }
}

// ❌ 나쁜 예
public sealed class BadService(IServiceProvider provider)
{
    public void DoWork()
    {
        var service = provider.GetRequiredService<IMyService>();
    }
}

// ✅ 좋은 예
public sealed class GoodService(IMyService myService)
{
    public void DoWork() { }
}