Design Patterns: Null Object

‘If you have a hammer, everything looks like a nail’. My favorite pattern today is the Null object pattern, so I’m applying it everywhere
But what’s this pattern exactly and why is it so useful? Let’s consider the following code :

class DiscountRepository
{
    public Discount GetDiscountFor(Order order)
    {
    }
}

public abstract class Discount
{   
   public abstract int Calculate(Order order);
}

We have an abstract class that represents our object and a repository.

Now what if you have an order for which there is no discount information available in the database. You always have to check for null and having this all over the place make the code less readable.

public void ProcessOrder(Order order)
{
    var discount=_repository.GetDiscountFor(order);
    if (discount != null)
        discount.Calculate(order);
}

I don’t like this code.  So let’s apply the Null Object pattern to make the above code prettier and less error-prone. All we need to do is to implement the abstract class(you can achieve the same result by using an interface) one more time but this time we will do it inside our Discount class like this :

public abstract class Discount
{
    public abstract int Calculate(Order order);
        
    public static readonly Discount NULL = new NullDiscount();
    private class NullDiscount : Discount
    {            
        public override int Calculate(Order order) { return 0; }
    }
}

Our ProcessOrder now looks like this :

public void ProcessOrder(Order order)
{
    var discount=_repository.GetDiscountFor(order);
    discount.Calculate(order);
}

And our GetDiscountFor method looks like this :

public Discount GetDiscountFor(Order order)
{
    if(/* Not Found */)
        return Discount.NULL;
    else
        /* The db record */
}