Unknown NullPointerException

Question

Hy!

I want to make a cardgame in c# The debugger always throws a NullPointerException. But the var card isn't null (it has a value shown during the debugging) The problem have to be in cards.add().

My Code:

namespace Uno_Logic
{
    class CardStack
    {
        private List<Card> cards;
        public void Cardstack()
        {
            cards = new List<Card>();
        }
        public void fillCardstack ()
        {
            for (Card_Value value = Card_Value.One; value <= Card_Value.DrawTwo; value++)
            {
                for (Card_Colour colour = Card_Colour.Yellow; colour < Card_Colour.Black; colour++)
                {
                    Card card = new Card(colour, value);
                    Card card2 = new Card(colour, value);
                    cards.Add(card); //**here throws the debugger the Exception**
                    cards.Add(card2);
                }

            }
         }
      }
   }

Please help!

Is this the real code? What does the debugger say about `cards` at that line? — Henk Holterman, May 05 '11 at 13:10
cards is null but my calling is cardstack = new CardStack(); cardstack.fillCardstack(); — user547995, May 05 '11 at 13:13
@user54, You do _not_ have a constructor. See answers below. — Henk Holterman, May 05 '11 at 13:15
@user547995 - Make the `public void Cardstack()` to be `public CardStack()` and you will be fine. — Petar Minchev, May 05 '11 at 13:15
Does this answer your question? [What is a NullReferenceException, and how do I fix it?](https://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Charlieface, Mar 04 '21 at 14:28

mBotros · Accepted Answer · 2011-05-05T13:19:27.600

6

replace:

    public void CardStack()
    {
        cards = new List<Card>();
    }

with

    public CardStack()
    {
        cards = new List<Card>();
    }

constructors in c# has no return type

edited May 05 '11 at 13:19

answered May 05 '11 at 13:11

mBotros

1,674
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Made the `s` letter capital to match the class name. – Petar Minchev May 05 '11 at 13:17

score 4 · Answer 2 · answered May 05 '11 at 13:11

4

I think your method Cardstack really should be a constructor:

public CardStack()
{
    cards = new List<Card>();
}

answered May 05 '11 at 13:11

Daniel Hilgarth

159,901
39
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score 2 · Answer 3 · answered May 05 '11 at 13:09

2

Then your cards variable is null. You haven't initalized it, because you probably haven't called the Cardstack() method. By the way it is better the cards = new List<Card>(); to be in a constructor.

answered May 05 '11 at 13:09

Petar Minchev

44,805
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I think `void CardStack()` was meant to _be_ the constructor. I overlooked the `void` – Henk Holterman May 05 '11 at 13:12

score 0 · Answer 4 · answered May 05 '11 at 13:10

0

This is not the card variable which is null when the exception is throwing : this is the cards variable which is null. Are you sure to call Cardstack() before calling fillCardstack() ?

answered May 05 '11 at 13:10

mnflz

66
1
3

score 0 · Answer 5 · answered May 05 '11 at 13:12

I think you wanted to declare a constructor for your CardStack class and initialize cards there.
If it's so, just remove 'void' before the CardStack method declaration, so it will become a constructor:

    public Cardstack() {
        cards = new List<Card>();
    }

Unknown NullPointerException

5 Answers5