How can I instantiate a base class and then convert it to a derived class?

Question

I was wondering how to do this, consider the following classes

public class Fruit
{
    public string Name { get; set; }
    public Color Color { get; set; }
}
public class Apple : Fruit
{
    public Apple()
    {

    }
}

How can I instantiate a new fruit but upcast to Apple, is there a way to instantiate a bunch of Fruit and make them apples with the name & color set. Do I need to manually deep copy?

Of course this fails

Fruit a = new Fruit();
a.Name = "FirstApple";
a.Color = Color.Red;
Apple wa = a as Apple;
System.Diagnostics.Debug.Print("Apple name: " + wa.Name);

Do I need to pass in a Fruit to the AppleCTor and manually set the name and color( or 1-n properties) Is there an better design to do this?

Answer 1

By default, the a instance is an instance of Fruit and is not convertible to Apple. It's not an Apple instance, and will never be.

You can, however, implement an explicit cast.

Answer 2

In your model, a Fruit is not an Apple - if you want an Apple, instantiate one. Alternatively, if you want to create an Apple using a Fruit as a starting point, create a constructor for Apple that can accept a Fruit.

You should note, that object models where intermediate classes in the inheritance hierarchy are instantiable (not abstract) is often a bad idea. In some cases, such hierarchies may be appropriate - but I've found that in general they are more trouble than they are worth. Think really hard about whether both Fruit and Apple need to be concrete and instantiable.