How to use a concrete class method created from an interface?

Question

In class Lawyer implemented all Interface properties and methods Ipessoafisica, and some other class-specific properties and methods Lawyer, how to Advocate() and Oab.

How to access this method? Since in the creation of the object Lawyer i created as Testemethod below:

public interface IPessoaFisica
{
    int PessoaId { get; set; }
    string Nome { get; set; }
    IEndereco Endereco { get; set; }
    IContato Contato { get; set; }
    string Cpf { get; set; }
    string Identidade { get; set; }
    char Sexo { get; set; }
}

//Classe Concreta
public class Advogado: IPessoaFisica
{
    public int PessoaId { get; set; }
    public string Nome { get; set; }
    public IEndereco Endereco { get; set; }
    public IContato Contato { get; set; }
    public char Sexo { get; set; }
    public string Identidade { get; set; }
    public string Cpf { get; set; }
    public string Oab { get; set; }

    public void Advogar()
    {
        //Fazer qualquer coisa
    }
}

public void TestMethod1()
{
    IPessoaFisica adv = new Advogado();
    adv.Cpf = "111.111.111-11"; //OK
    adv.Nome = "José da Silva"; //OK
    adv.Advogar(); //ERRO - IPessoaFisica não contém definição de Advogar.
               // Método esta na classe concreta Advogado,e não está declarado na interface IPessoaFisica.
}

Answer 1

As much as your object is built as Advogado he is declared as IPessoaFisica which makes accessible only the interface methods IPessoaFiscia. To gain access to the method Advogar you will need to make one of the following changes:

Option 1- Declare method advocate on interface IPessoaFisica

Option 2- Change the type of the Adv variable to Advogado:

public void TestMethod1()
{
    Advogado adv = new Advogado();
    adv.Cpf = "111.111.111-11";
    adv.Nome = "José da Silva";
    adv.Advogar();
}

Interfaces are used to allow building common abstractions of objects and thus use polymorphism. This is only useful when polymorphic methods are declared in the interface because then you can operate on variables declared with the type of interfaces. When you need to use specific methods that cannot be abstracted and consequently cannot be declared in the interfaces it is necessary to operate on variables with the type of concrete implementation to allow access to its concrete methods.