Question

How can I do something like the following?
My A object is null after calling GetB even though A inherits from B.

class Program
{
    public class A : B
    {
    }

    public class B
    {
    }

    static void Main(string[] args)
    {
        A a = GetB() as A;
        Console.WriteLine(a == null); // it is null!

        Console.WriteLine("Console.ReadKey();");
        Console.ReadKey();
    }
    public static B GetB()
    {
        return new B();
    }
}

Answer 1

You’re trying to downcast your B into an A. You can’t do that, nor does it makes sense because we do not know if a B is going to be an A. It would be better to build a constructor in your A class that takes a B as a parameter.

public class A : B
{
    public A(B b)
    {
        //perform your conversion of a B into an A
    }
}

public class B
{
    public B(){}
}

static void Main(string[] args)
{
    B b = new B();
    A a = new A(b);
    Console.WriteLine(a == null); // it is null!

    Console.WriteLine("Console.ReadKey();");
    Console.ReadKey();
}