Object-Oriented Design Principles

public class EmailSender
{
  public void Send(string message) { ... }
}

public class NotificationService
{
  private readonly EmailSender _emailSender = new();

  public void Notify(string message) => _emailSender.Send(message);
}

public interface IMessageSender
{
  void Send(string message);
}

public class EmailMessageSender : IMessageSender
{
  public void Send(string message) { ... }
}

public class NotificationService(IMessageSender messageSender)
{
  public void Notify(string message) => messageSender.Send(message);
}

public class Bird
{
  public virtual void Fly() { ... }
}

public class Penguin : Bird
{
  public override void Fly() => throw new NotSupportedException("Penguins can't fly");
}

public interface IBird
{
  void Eat();
}

public interface IFlyingBird : IBird
{
  void Fly();
}

public class Sparrow : IFlyingBird
{
  void Eat() { ... }
  void Fly() { ... }
}

public class Penguin : IBird
{
  void Eat() { ... }
}

public class PaymentProcessor
{
  public void Process(string type, double amount)
  {
    if (type == "creditCard")
    {
      // Credit card logic
    }
    else if (type == "paypal")
    {
      // PayPal logic
    }

    // Adding crypto means modifying this method.
  }
}

public class Report
{
  public string GenerateContent() { ... }
  public void PrintToPdf() { ... }
  public void SaveToFile() { ... }
}

public class Report
{
  public string GenerateContent() { ... }
}

public class PdfPrinter
{
  public void Print(Report report) { ... }
}

public class FileStorage
{
  public void Save(string content) { ... }
}