The Cost of DI: 4 different versions of the same code, demonstrating the cost of added indirection

1-Simplest.cs

public class SpeakerController : Controller
{
  public ActionResult Create(int presentationId, FormCollection data)
  {
    var presentation = Presentation.Find(presentationId);
    var vm = SpeakerNewViewModel.Build(presentation);

    UpdateModel(vm, data);

    var presentationApi = new PresentationApi(presentationId);
    presentationApi.AddSpeaker(vm.Speaker);

    return View();
  }
}

public class PresentationApi
{
  int presentationId;
  PresentationSpeakers presentationSpeakers;
  OrdersSpeakers ordersSpeakers;

  public PresentationApi(int presentationId)
  {
    this.presentationId = presentationId;
    ordersSpeakers = new OrdersSpeakers(presentationId);
  }

  public void AddSpeaker(Speaker speaker)
  {
    LoadPresentationSpeakersRecord();
    speaker.PresentationId = presentationId;

    presentationSpeakers.AddSpeaker(speaker);
    ordersSpeakers.AddSpeaker(speaker);

    speaker.Save();
    presentationSpeakers.Save();
  }

  public void RemoveSpeaker(Speaker speaker)
  {
    // ... very similar to AddSpeaker ...
  }

  void LoadPresentationSpeakersRecord()
  {
    presentationSpeakers = PresentationSpeakers.FindByPresentationId(presentationId);
  }
}

2-WithIPresentationApi.cs

public class Modules
{
  public void Register(IContainer container)
  {
    container.Register<IPresentationApi>.To<PresentationApi>();
  }
}

public class SpeakerController : Controller
{
  IPresentationApi presentationApi;

  public SpeakerController(IPresentationApi presentationApi)
  {
    this.presentationApi = presentationApi;
  }

  public ActionResult Create(int presentationId, FormCollection data)
  {
    var presentation = Presentation.Find(presentationId);
    var vm = SpeakerNewViewModel.Build(presentation);

    UpdateModel(vm, data);

    presentationApi.AddSpeaker(presentationId, vm.Speaker);

    return View();
  }
}

public interface IPresentationApi
{
  void AddSpeaker(int presentationId, Speaker speaker);
  void RemoveSpeaker(int presentationId, Speaker speaker);
}

public class PresentationApi : IPresentationApi
{
  public void AddSpeaker(int presentationId, Speaker speaker)
  {
    var presentationSpeakers = LoadPresentationSpeakersRecord(presentationId);
    speaker.PresentationId = presentationId;

    presentationSpeakers.AddSpeaker(speaker);

    var ordersSpeakers = new OrdersSpeakers(presentationId);
    ordersSpeakers.AddSpeaker(speaker);

    speaker.Save();
    presentationSpeakers.Save();
  }

  public void RemoveSpeaker(int presentationId, Speaker speaker)
  {
    // ... very similar to AddSpeaker ...
  }

  PresentationSpeakers LoadPresentationSpeakersRecord(int presentationId)
  {
    return PresentationSpeakers.FindByPresentationId(presentationId);
  }
}

3-WithIOrdersSpeakers.cs

public class Modules
{
  public void Register(IContainer container)
  {
    container.Register<IPresentationApi>.To<PresentationApi>();
    container.Register<IOrdersSpeaker>.To<OrdersSpeaker>();
  }
}

public class SpeakerController : Controller
{
  IPresentationApi presentationApi;

  public SpeakerController(IPresentationApi presentationApi)
  {
    this.presentationApi = presentationApi;
  }

  public ActionResult Create(int presentationId, FormCollection data)
  {
    var presentation = Presentation.Find(presentationId);
    var vm = SpeakerNewViewModel.Build(presentation);

    UpdateModel(vm, data);

    presentationApi.AddSpeaker(presentationId, vm.Speaker);

    return View();
  }
}

public interface IPresentationApi
{
  void AddSpeaker(int presentationId, Speaker speaker);
  void RemoveSpeaker(int presentationId, Speaker speaker);
}

public interface IOrdersSpeakers
{
  void AddSpeaker(int presentationId, Speaker speaker);
  void RemoveSpeaker(int presentationId, Speaker speaker);
}

public class PresentationApi : IPresentationApi
{
  IOrdersSpeakers ordersSpeakers;

  public PresentationApi(IOrdersSpeakers ordersSpeakers)
  {
    this.ordersSpeakers = ordersSpeakers;
  }

  public void AddSpeaker(int presentationId, Speaker speaker)
  {
    var presentationSpeakers = LoadPresentationSpeakersRecord(presentationId);
    speaker.PresentationId = presentationId;

    presentationSpeakers.AddSpeaker(speaker);
    ordersSpeakers.AddSpeaker(presentationId, speaker);

    speaker.Save();
    presentationSpeakers.Save();
  }

  public void RemoveSpeaker(int presentationId, Speaker speaker)
  {
    // ... very similar to AddSpeaker ...
  }

  PresentationSpeakers LoadPresentationSpeakersRecord(int presentationId)
  {
    return PresentationSpeakers.FindByPresentationId(presentationId);
  }
}

4-WithRepositories.cs

public class Modules
{
  public void Register(IContainer container)
  {
    container.Register<IPresentationApi>.To<PresentationApi>();
    container.Register<IOrdersSpeaker>.To<OrdersSpeaker>();
    container.Register<IPresentationRepository>.To(...);
    container.Register<IPresentationSpeakersRepository>.To(...);
  }
}

public interface IPresentationRepository
{
  ...
}

public interface ISpeakerRepository
{
  ...
}

public interface IPresentationSpeakersRepository
{
  ...
}

public class SpeakerController : Controller
{
  IPresentationApi presentationApi;
  IPresentationRepository presentationRepo;

  public SpeakerController(
      IPresentationApi presentationApi,
      IPresentationRepository presentationRepo)
  {
    this.presentationApi = presentationApi;
    this.presentationRepo = presentationRepo;
  }

  public ActionResult Create(int presentationId, FormCollection data)
  {
    var presentation = presentationRepo.Find(presentationid);
    var vm = SpeakerNewViewModel.Build(presentation);

    UpdateModel(vm, data);

    presentationApi.AddSpeaker(presentationId, vm.Speaker);

    return View();
  }
}

public interface IPresentationApi
{
  void AddSpeaker(int presentationId, Speaker speaker);
  void RemoveSpeaker(int presentationId, Speaker speaker);
}

public interface IOrdersSpeakers
{
  void AddSpeaker(int presentationId, Speaker speaker);
  void RemoveSpeaker(int presentationId, Speaker speaker);
}

public class PresentationApi : IPresentationApi
{
  IOrdersSpeakers ordersSpeakers;
  ISpeakerRepository speakerRepo;
  IPresentationSpeakersRepository presentationSpeakerRepo;

  public PresentationApi(
      IOrdersSpeakers ordersSpeakers,
      ISpeakerRepository speakerRepo,
      IPresentationSpeakersRepository presentationSpeakersRepo)
  {
    this.ordersSpeakers = ordersSpeakers;
    this.speakerRepo = speakerRepo;
    this.presentationSpeakerRepo = presentationSpeakerRepo;
  }

  public void AddSpeaker(int presentationId, Speaker speaker)
  {
    var presentationSpeakers = LoadPresentationSpeakersRecord(presentationId);
    speaker.PresentationId = presentationId;

    presentationSpeakers.AddSpeaker(speaker);
    ordersSpeakers.AddSpeaker(presentationId, speaker);

    speakerRepo.Save(speaker);
    presentationSpeakerRepo.Save(presentationSpeakers);
  }

  public void RemoveSpeaker(int presentationId, Speaker speaker)
  {
    // ... very similar to AddSpeaker ...
  }

  PresentationSpeakers LoadPresentationSpeakersRecord(int presentationId)
  {
    presentationSpeakerRepo.FindByPresentationId(presentationId);
  }
}