Question

Given that I have an instance of Fruit with some properties set, and I want to get those properties into a new Pear instance (because this particular Fruit happens to have the qualities of a pear), what’s the best way to achieve this effect?

For example, what we can’t do is simply cast a Fruit to a Pear, because not all Fruits are Pears:

public static class PearGenerator {
    public static Pear CreatePear () {

        // Make a new generic fruit.
        Fruit genericFruit = new Fruit();

        // Upcast it to a pear. (Throws exception: Can't cast a Fruit to a Pear.)
        Pear pear = (Pear)genericFruit;

        // Return freshly grown pear.
        return ( pear );
    }
}

public class Fruit {
    // some code
}

public class Pear : Fruit {

    public void PutInPie () {
        // some code
    }

}

Thanks!

Update:

I don’t control the “new Fruit()” code. My starting point is that I’ve got a Fruit to work with. I need to get that Fruit into a new Pear somehow. Maybe copy all the properties one by one?

Answer 1

If you’re not responsible for Fruit but are the author of Pear, then maybe a different design would help:

It may be better to change your Pear class to more of a FruitWrapper class. This class would hold a reference to a Fruit instead of being a more specific kind of Fruit. It can have its own functionality (the reason you don’t want to use Fruit in the first place), and pass on fruit-type functionality to the Fruit it’s got inside. If it needs to work with library code that expects a Fruit, just pass the one it’s holding on to…