Decorator Pattern in object-oriented programming

I was studying the "Decorator pattern". Doing some tests with C# I do not understand why I do not get the expected result. This is the code:

public abstract class Drink
{
    public string description = "Generic drink";

    public string GetDescription()
    {
        return description;
    }
}

public abstract class DrinkDecorator : Drink
{
    public abstract string GetDescription();
}


public class SecretIngredient : DrinkDecorator
{
    Drink drink;

    public SecretIngredient (Drink drink)
    {
        this.drink = drink;
    }

    public override string GetDescription()
    {
        return this.drink.GetDescription() + ", SecretIngredient ";
    }
}

public class Espresso : Drink
{
    public Espresso()
    {
        description = "Espresso";
    }
}


[TestFixture]
class TestClass
{
    [Test]
    public void TestMethod()
    {
        Drink drink = new Espresso();
        System.Diagnostics.Debug.WriteLine(drink.GetDescription());

        drink = new SecretIngredient (drink);
        System.Diagnostics.Debug.WriteLine(drink.GetDescription());
    }
}

Performing the test I get:

Espresso

Generic drink

While I would have expected:

Espresso

Espresso, SecretIngredient

Why? Thanks in advance.