service registration

public interface ICacheService
{
    public void SetSomething(object something);
    public object GetSomething();
}

public class CacheService : ICacheService
{
    private object _something;

    public void SetSomething(object something) => _something = something;

    public object GetSomething() => _something;
}

public class Service()
{
    private ICacheService _serviceCache;

    public Service(ICacheService cache)
    {
        _serviceCache = cache;
    }

    public void DoSomething(object something)
    {
        //stuff

        _cacheService.SetSomething(something);

        //other stuff
    }

    public object ReturnSomething()
    {
        return _cacheService.GetSomething();
    }
}

public class Program{

    private ICacheService _cacheService;
    private Service _service;

    public static int Main(string[] args)
    {
        // inizializzo un service provider ecc...
        IServiceProvider provider = /*serviceProvider*/ ;

        _cacheService = new CacheService();

        RegisterServices(provider);

        DoingSomethingWithServices(provider);

        return 0;
    }

    private void RegisterServices(IServiceProvider provider)
    {
        provider.Services.AddSingleton<ICacheService>(_cacheService);
        provider.Services.AddTransient<Service>();
    }

    private void DoingSomethingWithServices(IServiceProvider provider)
    {
        Service serviceInstance1 = provider.GetService<Service>();
        Service serviceInstance2 = provider.GetService<Service>();
        Service serviceInstance3 = provider.GetService<Service>();

        object something = 1;
        serviceInstance1.DoSomething(something);

        object something = 2;
        serviceInstance2.DoSomething(something);

        object something = 3;
        serviceInstance1.DoSomething(something);

        List<object> somethings = new List<object>();

        somethings.Add(serviceInstance1.ReturnSomething())
        somethings.Add(serviceInstance2.ReturnSomething())
        somethings.Add(serviceInstance3.ReturnSomething())

        // what's inside somethings? 3,3,3 ? or 1,2,3?
    }
}