Null reference exception when filling an multidimensional image array

Question

I create the Array cardImages in Class KartenClass

public Image[][][] cardImages = new Image[9][][];

I wrote a method called arrbef() to fill it

public void arrbef()
{ 
    this.cardImages[0][0] = new Image[3] { global::WindowsFormsApplication4.Properties.Resources.Card, global::WindowsFormsApplication4.Properties.Resources.CardBack, global::WindowsFormsApplication4.Properties.Resources.CardSet };
    this.cardImages[0][1] = new Image[3] { global::WindowsFormsApplication4.Properties.Resources.Card, global::WindowsFormsApplication4.Properties.Resources.CardBack, 
etc....

and in my Form i call the method arrbef and try to fill it.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;


namespace WindowsFormsApplication4
{
    public partial class Karten : Form
    {
        KartenClass karten = new KartenClass();
        int standort = 0;

        public Karten()
        {
            InitializeComponent();
            KartenClass.karten[0].arrbef();
        }

But when i click on the button which links to this form, i get the following Error:

System.NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.
   bei WindowsFormsApplication4.KartenClass.arrbef() in c:\Users\david.kresse\Documents\Visual Studio 2012\Projects\WindowsFormsApplication5\KartenClass.cs:Zeile 24.
   bei WindowsFormsApplication4.Karten..ctor() in c:\Users\david.kresse\Documents\Visual Studio 2012\Projects\WindowsFormsApplication5\Karten.cs:Zeile 22.
   bei WindowsFormsApplication4.Start.btnStartGoToKarten_Click(Object sender, EventArgs e) in c:\Users\david.kresse\Documents\Visual Studio 2012\Projects\WindowsFormsApplication5\Start.cs:Zeile 394.
   bei System.Windows.Forms.Control.OnClick(EventArgs e)
   bei System.Windows.Forms.Button.OnClick(EventArgs e)
   bei System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   bei System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   bei System.Windows.Forms.Control.WndProc(Message& m)
   bei System.Windows.Forms.ButtonBase.WndProc(Message& m)
   bei System.Windows.Forms.Button.WndProc(Message& m)
   bei System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   bei System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   bei System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

What did i do wrong? I did exactly the same with another array (which was one dimensonal).

I hope you can help.

It's always best to turn on break on throw in the debugger settings. Then you can get a callstack when the problem happens, and you can learn how to debug it yourself. — C.J.
– C.J., Commented Dec 10, 2013 at 13:31
Welcome to Stack Overflow! Almost all cases of NullReferenceException are the same. Please see "What is a NullReferenceException in .NET?" for some hints. — John Saunders
– John Saunders, Commented Dec 10, 2013 at 14:03

Jon Skeet · Accepted Answer · 2013-12-10 13:34:00Z

1

This:

public Image[][][] cardImages = new Image[9][][];

... creates a top level array with 9 elements in. Every element value is null. You need:

for (int i = 0; i < cardImages.Length; i++) {
    cardImages[i] = new Image[???][]; // What length do you want?
}

Then you can populate cardImages[0][0] etc as you're doing.

Personally I'd try to avoid 3-dimensional arrays (or arrays of arrays of arrays in this case) - it can get messy. It may be appropriate in this case though; it's hard to say without more information.

EDIT: With more information, it may make sense to model this as a Category[] (or List<Category>) where a Category has a Card[] or List<Card>, and a Card has an Image[] or List<Image>. Then at the top level you just have a collection of categories.

edited Dec 10, 2013 at 13:34

answered Dec 10, 2013 at 13:28

Jon Skeet

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4 Comments

user3086972 Over a year ago

the array contains 9 categories with 20 cards, each with 3 conditions.

Jon Skeet Over a year ago

@user3086972: See my edit for how I'd model it then - by making the meaning of each level clear, the whole code becomes clearer.

user3086972 Over a year ago

Okay thank you, i will try that. I think i will have to hit a few tutorials for that though. But i´m on it! Big thanks to you my friend! :)

user3086972 Over a year ago

I tried it with the for-loop and it works perfectly. I will try it again with lists as soon as i learned how to do it. :)

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