Question

Can someone tell me if this class structure is bad?

class abstract Parent{
  public Child Foo(){
    return new Child();
  }
}

class Child : Parent{}

I’ve heard that referring to a derived type from a base type is always bad and signs of bad design. Can someone tell me why this is bad or even if it is bad?

Answer 1

To elaborate on dkackman’s answer, your factory ideally would return objects of child types, but declared as the parent type.

class Factory
{
    public Parent Foo()
    {
        return new Child();
    }

    public Parent Bar()
    {
        return new OtherChild();
    }
}

The basic idea is that your calling code shouldn’t care which child class it gets back. This is one concept of the Liskov Substitution Principle.