How to cast a typed class in C#?

Question

I own the class Carro who inherits from the class Objeto and another class CarroTeste implementing the abstract class TesteBase that by its implements the interface ITeste, both (TesteBase and ITeste) typed with restriction to use only class objects Objeto, the problem occurs when I try to instantiate a CarroTeste in an Iteste variable ITeste<Objeto> teste = new CarroTeste();, IDE says I can’t do an implicit type conversion. The class code is below.

public class Objeto
{
    protected string _nome;
    public string nome { get{return _nome;} }
}

public class Carro : Objeto
{
    public Carro()
    {
        _nome = "Carro";
    }
}

public interface ITeste<T> where T : Objeto
{
    string GetNome();
    List<T> List { get; }
    T GetChange(T obj);
}

public abstract class TesteBase<T> : ITeste<T> where T : Objeto
{
    protected Objeto _obj = null;
    public abstract string GetNome();
    public abstract List<T> List { get; }
    public abstract T GetChange(T obj);
}

public class CarroTeste : TesteBase<Carro>
{
    public override string GetNome()
    {
        return "Meu nome é : " + _obj.nome;
    }

    public override List<Carro> List
    {
        get { return new List<Carro>(); }
    }

    public override Carro GetChange(Carro obj)
    {
        return obj;
    }
}

Answer 1

Are you in trouble variance. To solve you must allow the interface to be used covariantly. See:

public class Program {
    public static void Main() {
        ITeste<Objeto> teste = new CarroTeste();
    }
}

public class Objeto {
    protected string _nome;
    public string nome => _nome;
}

public class Carro : Objeto {
    public Carro() => _nome = "Carro";
}

public interface ITeste<out T> where T : Objeto {
    string GetNome();
}

public abstract class TesteBase<T> : ITeste<T> where T : Objeto {
    protected Objeto _obj = null;
    public abstract string GetNome();
}

public class CarroTeste : TesteBase<Carro> {
    public override string GetNome() => "Meu nome é : " + _obj.nome;
}

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

Note the line public interface ITeste<out T> where T : Objeto { using the modifier out to indicate the variance.

I don’t know if it solves the way you want it to, but it solves the immediate problem.

Answer 2

Your code is correct, the . NET just doesn’t do the implicit conversion (without you asking).

Explicitly convert the object:

ITeste<Objeto> teste = (ITeste<Objeto>) new CarroTeste();

That one (tipo) instructs the conversion of what comes next. In this case, it is what we call explicit conversion.

This behavior exists to avoid conversions without the programmer wanting. If that happened and caused any trouble, it would be very, very difficult to trace.