Polymorphism or inheritance?

Question

I’m learning about polymorphism and I’ve been wondering if what I’m doing is actually polymorphism or just inheritance?

If it’s polymorphism what’s in my code that I can clearly identify is polymorphism?

If it is not, what could be done to be polymorphism?

public abstract class Pessoa
{
    public abstract int Id { get; set; }
    public abstract string Nome { get; set; }

    public abstract void Andar();
}

public class Comprador : Pessoa
{
    public override int Id { get; set; }  
    public override string Nome { get; set; }      

    public override void Andar()
    {
        throw new NotImplementedException();
    }

    public void Comprar()
    {

    }
}

public class Vendedor : Pessoa
{
    public override int Id { get; set; }
    public override string Nome { get; set; }

    public override void Andar()
    {
        throw new NotImplementedException();
    }

    public void Vender()
    {

    }
}

public class Sistema
{
    public void MeuMetodoTeste()
    {
        Comprador pessoaComprador = new Comprador();
        pessoaComprador.Comprar();

        Vendedor pessoaVendedor = new Vendedor();
        pessoaVendedor.vender();
    }
}

Answer 1

You’re doing both. When you put Comprador : Pessoa or Vendedor : Pessoa is making inheritance, ie the first name is defining a class that will be composed initially by the class referenced in the second name. Is Comprador and Vendedor will be subclasses and subtypes of Pessoa.

These two concrete classes can be used in the place where the abstract class is expected Pessoas, so in this case polymorphism will occur. In this case it will actually occur elsewhere in the code. It did not occur in your example because there did not need to be a replacement. These classes were used to create objects of the declared type itself and were not used elsewhere.

You can see that the polymorphism will occur more clearly in the methods declared in Pessoa. Since they are all abstract, they must be implemented in the descending classes. And for these implementations to be used a polymorphic mechanism needs to be adopted.

Here begins to occur polymorphism concretely:

public class Sistema {
    public void MeuMetodoTeste() {
        var pessoaComprador = new Comprador();
        pessoaComprador.Comprar();
        UmOutroMetodo(pessoaComprador);
        var pessoaVendedor = new Vendedor();
        pessoaVendedor.vender();
        UmOutroMetodo(pessoaVendedor);
    }
    //note que se passar um objeto do tipo Pessoa nem funcionaria de fato, deve ser conreto
    public void UmOutroMetodo(Pessoa pessoa) {
        Console.WriteLine(pessoa.Nome); //vai pegar o que foi usado na classe concreta
        pessoa.Andar(); //vai lançar a exceção
    }
}

Behold working in the ideone. And in the .NET Fiddle. Also put on the Github for future reference.

Note that polymorphism occurs concretely when there is a virtual method (abstract methods are virtual by definition). A virtual method can be overwritten (abstracts must, after all, have no implementation). Note that there can only be substitution if the method has exactly the same signature. Substitution takes place through the keyword override.

This can be better explained in a more general sense in other questions (I have answered this in different ways in different locations - have better examples to show polymorphism in these questions):

• When to use polymorphism
• What are the differences between the concepts of polymorphism and superscript in OOP?
• How "new" and "virtual" work in C#?
• This is polymorphism?
• Doubt about Inheritance
• Principle of Liskov’s replacement
• Classes that implement interfaces are considered subclasses? (note that this example Pessoa could be an interface)
• Microsoft reference
• The most voted question in the SO on the subject