Question

Is it possible to remove a decorator from an object?

Say I have the following code:

abstract class Item
{
    decimal cost();
}

class Coffee : Item
{
    decimal cost()
    { // some stuff }
}

abstract class CoffeeDecorator : Item
{
    Item decoratedItem;
}

class Mocha : CoffeeDecorator 
{
    Item decoratedItem;

    public Mocha(Coffee coffee)
    {
       decoratedItem = coffee;
    }
}

public void Main(string[] args)
{
    Item coffeeDrink = new Mocha(new Coffee());
}

Is there a way to remove the “new Mocha()” from my new “coffee” object?

EDIT: Clarification – I want to be able to remove just ONE decorator, not all of them. So if I had a Mocha decorator AND a Sugar decorator on the Coffee object, I want to know if I can remove just the “Mocha” decorator.

Answer 1

First, this assignment is not legal:

Coffee coffee = new Mocha(new Coffee());

A Mocha is not a Coffee nor is there an implicit cast from a Mocha to a Coffee. To “remove” the decorator, you need to provide either a method or a cast to do so. So you could add an undecorate method to Mocha:

public Coffee Undecorate() {
    return (Coffee)decoratedItem;
}

Then you could say

Coffee coffee = new Mocha(new Coffee()).Undecorate();

Alternatively, you could provide an implicit cast operator in the Mocha class:

public static implicit operator Coffee(Mocha m) {
    return (Coffee)m.decoratedItem;
}

Then your line

Coffee coffee = new Mocha(new Coffee());

would be legal.

Now, your question suggests a potential misunderstanding of the design pattern (and, in fact, your implementation suggests one too). What you’re trying to do is very smelly. The right way to go about using the decorator pattern is like so. Note that CoffeeDecorator derives from Coffee!

abstract class Item { public abstract decimal Cost(); }
class Coffee : Item { public override decimal Cost() { return 1.99m; } }
abstract class CoffeeDecorator : Coffee {
    protected Coffee _coffee;
    public CoffeeDecorator(Coffee coffee) { this._coffee = coffee; }
}
class Mocha : CoffeeDecorator {
    public Mocha(Coffee coffee) : base(coffee) { }
    public override decimal Cost() { return _coffee.Cost() + 2.79m; }
}
class CoffeeWithSugar : CoffeeDecorator {
    public CoffeeWithSugar(Coffee coffee) : base(coffee) { }
    public override decimal Cost() { return _coffee.Cost() + 0.50m; }
}

Then you can say:

Coffee coffee = new Mocha(new CoffeeWithSugar(new Coffee()));
Console.WriteLine(coffee.Cost()); // output: 5.28

Given this, what do you need to undecorate it for?