Can I pass arguments to a base constructor from a derived class's default constructor?

Question

Suppose I have an abstract base class Deck:

public abstract class Deck
{
   public List<Card> cards;

   public Deck(string[] values, string[] suits)
   {...}

   ...
}

and a derived class EuchreDeck:

public class EuchreDeck : Deck
{
   string[] values = new string[] { "9", "10", "J", "Q", "K", "A" };
   string[] suits = new string[] { "clubs", "spades", "hearts", "diamonds" };

   public EuchreDeck() : base(values, suits) // Error.
   {}

   ...
}

I want the ability to instantiate EuchreDeck and have the two string arrays passed to the base class, i.e. var gameDeck = new EuchreDeck();.

Currently I'm getting the error: "An object reference is required for the non-static field, method, or property EuchreDeck.values."

Is this possible, or will calling the derived default constructor always call the base default constructor?

Answer 1

Yes, you can do this if you make the arrays static:

public class EuchreDeck : Deck
{
   private static readonly string[] values = new string[] { "9", "10", "J", "Q", "K", "A" };
   private static readonly string[] suits = new string[] { "clubs", "spades", "hearts", "diamonds" };

   public EuchreDeck() : base(values, suits)
   {

   }
}

The reason why you can't use it as you had with instance-level members is because it's not legal to do so. This comes from the C# specification 10.10.1 Constructor Initializers where it states:

An instance constructor initializer cannot access the instance being created. Therefore it is a compile-time error to reference this in an argument expression of the constructor initializer, as is it a compile-time error for an argument expression to reference any instance member through a simple-name.

By switching the arrays to be static, they are no longer accessed via the instance but rather by the EuchreDeck type.

---

That said, I might suggest you take a slight tweak on the design. Maybe use a factory to create these specialized decks for you rather than their constructors.

As an example, maybe refactor something like this:

Change your base Deck to just take the set of cards:

public abstract class Deck
{
    public List<Card> Cards;
    protected Deck(IEnumerable<Card> cards)
    {
        this.Cards = new List<Card>(cards);
    }
}

Then have the factory setup like this:

public class EuchreDeck : Deck
{
    private EuchreDeck(IEnumerable<Card> cards) : base(cards)
    {

    }

    public class Factory : DeckFactory
    { 
        private static readonly string[] Values = new string[] { "9", "10", "J", "Q", "K", "A" };
        private static readonly string[] Suits = new string[] { "clubs", "spades", "hearts", "diamonds" };

        public static EuchreDeck Create()
        {
            var cards = CreateCards(Values, Suits);
            return new EuchreDeck(cards);
        }
    }
}

Instantiation/usage as:

EuchreDeck.Factory.Create();

You could play around with the factory usage. I just nested it in the class so you couldn't create a EuchreDeck with an invalid set of cards. Your DeckFactory base would have your conversion method (which looks like you currently have in your Deck constructor)

Beyond that, I'm not sure if you have a specific need for a EuchreDeck; I'm assuming you have other methods associated with it? If not, you could probably ditch the class altogether and just let the factory create a Deck with the needed cards.

Answer 2

I think the problem is that the values and suits should be declared static.

static string[] values = new string[] { "9", "10", "J", "Q", "K", "A" };
static string[] suits = new string[] { "clubs", "spades", "hearts", "diamonds" };

That way, they will be available at when instantiating the new class.

Answer 3

You have to have somewhere that variable decalred :

public EuchreDeck() : base(values, suits) // Error.
{}

where are values and suits declared?

You have to define them static, like:

static readonly string[] values = new string[] { "9", "10", "J", "Q", "K", "A" };
static readonly string[] suits = new string[] 
              { "clubs", "spades", "hearts", "diamonds" }; 

The thing is that base(..) can only access values visible in ctor scope. So you can not pass instance variables, as instance is not accessible on that level, but you can pass ctor parameters and static members of calling class.