Interfaces

EmailNotifier.cs

public class EmailNotifier : INotifier
{
  public void Send(string subject, string body, Owner recipient)
  {
    // logic that sends email
  }
}

Feeding.cs

public class FeedingTime
{
  public void FeedPet(IEats pet)
  {
    var food = new Food();
    pet.eat(food);
  }
}

FeedingTimeWithEmail.cs

public class FeedingTime
{
  private EmailService _myEmailService;
  
  public FeedingTime()
  {
    _myEmailSerice = new EmailService(); // we probably have to pass in some SMTP details or something...
  }
  
  public void FeedPet(IEats pet)
  {
    var food = new Food();
    pet.eat(food);
    
    // assuming we have set up pet and owner properties for out pet...
    _myEmailService.Send("Your pet has been fed", $"Your pet {pet.name} has been fed. Kind Regards, FeedingTime", pet.Owner.EmailAddress);
  }
}

FeedingTimeWithINotifier.cs

public class FeedingTime
{
  private INotifier _notifier;
  
  public FeedingTime(INotifier notifier)
  {
    _notifier = notifier;
  }
  
  public void FeedPet(IEats pet)
  {
    var food = new Food();
    pet.eat(food);
    
    // assuming we have set up pet and owner properties for out pet...
    _notifier.Send("Your pet has been fed", $"Your pet {pet.name} has been fed. Kind Regards, FeedingTime", pet.Owner);
  }
}

Fish.cs

public interface ISwims
{
  void swim();
}

public class Fish : ISwims, IEats, ISleeps
{
  public void sleep()
  {
    // logic for when your fish is sleeping...
  }
  
  public void eat(Food food)
  {
    // logic for when your fish eats...
  }
  
  public void swim()
  {
    // logic for when your fish is swimming...
  }
}

GoldieMailNotifier.cs

public class EmailNotifier : INotifier
{
  private IGoldieEmail _goldieMail;
  
  public EmailNotifier(IGoldieEmail goldieMail)
  {
    _goldieMail = goldieMail;
  }
  
  public void Send(string subject, string body, Owner recipient)
  {
    _goldieMail.SendEmail(body, subjcet, new string[] { recipient.EmailAddress });
  }
}

GoldieStartup.cs

public class Startup
{
    // ommitted for clarity...
 
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddGoldieMail("senderAddress");
    }
    
    // ommitted for clarity...
}

IEats.cs

public interface IEats
{
  void eat(Food food);
}

IGoldieEmail.cs

public interface IGoldieEmail
{
  void SendEmail(string body, string subject, string[] recipients);
}

INotifier.cs

public interface INotifier
{
  void Send(string subject, string body, Owner recipient);
}

ISleeps.cs

public interface ISleeps
{
  void sleep();
}

pets.cs

public class Cat: IEats, ISleeps
{
  public void sleep()
  {
    // logic for when your cat is sleeping...
  }
  
  public void eat(Food food)
  {
    // logic for when your cat eats...
  }
}

public class Dog: IEats, ISleeps
{
  public void sleep()
  {
    // logic for when your dog is sleeping...
  }
  
  public void eat(Food food)
  {
    // logic for when your dog eats...
  }
}

Startup.cs

public class Startup
{
    // ommitted for clarity...
 
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddSingleton<INotifier, EmailNotifier>();
    }
    
    // ommitted for clarity...
}