SOLID [D] - Dependency inversion

solid-D-right-implementation.cs

public interface IPaymentMethod
{
  bool Pay(int amount);
}

public class CreditCard : IPaymentMethod
{
  public bool Pay(int amount)
  {
    // do something
  }  
}

public class DebitCard : IPaymentMethod
{
  public bool Pay(int amount)
  {
    // do something
  }  
}

public class Cash : IPaymentMethod
{
  public bool Pay(int amount)
  {
    // do something
  }  
}

public interface IPerson
{
  IPaymentMethod PaymentMethod { get; }
}

public class Person : IPerson
{
  public IPaymentMethod PaymentMethod { get; private set; }

  public Person(IPaymentMethod paymentMethod)
  {
    this.PaymentMethod = paymentMethod;
  }
}

// calling

IPaymentMethod creditCard = new CreditCard();
IPerson person = new Person(creditCard);

person.PaymentMethod.Pay(100);

IPaymentMethod cash = new Cash();
IPerson person2 = new Person(cash);

person2.PaymentMethod.Pay(100);