NullReferenceException when assigning property of array item

Question

I want to create an array of my class Word. Here is the code:

public static void Main(string[] args)
    {
        Word[] Wörter = new Word[5];
        Wörter[0]._Word_ = "Hallo";
        Wörter[1]._Word_ = "Flugzeug";
        Wörter[2]._Word_ = "Automobil";
        Wörter[3]._Word_ = "Musikanlage";
        Wörter[4]._Word_ = "Steuerung";


    }

 public class Word
    {
        private string _Word;
        private int _counter;

        public string _Word_
        {
            get
            {
                return _Word;
            }
            set
            {
                _Word = value; 
            }

        }

        public int _counter_
        {
            get
            {
                return _counter;
            }
            set
            {
                _counter = 0;
            }
        }
    }

I always get a System.NullReferenceException in this line

Wörter[0]._Word_ = "Hallo";

If I call the an instance of my class(without array), everything works fine. I think I got a problem with the array. Maybe is using an array in this situation not the best think to do. Thank you for helping!

I am not sure what you are expecting. You have to create the instances somewhere, i.e. `new Word()`. See [What is a NullReferenceException and how do I fix it?](http://stackoverflow.com/q/4660142/238902) for more info — default, Nov 28 '14 at 16:18
What is with the properties being surrounded with underscores? My eyes don't know what to do with this. — Drew Kennedy, Nov 28 '14 at 16:20
@DrewKennedy I don't often create classes with properties but I heard that you should avoid using public properties - Maybe the convention is wrong but that's just for testing things.. — avbmath, Nov 28 '14 at 16:24

score 5 · Answer 1 · answered Nov 28 '14 at 16:17

5

You only istantiate the array but not the elements

Word[] Wörter = new Word[5];
Wörter[0] = new Word { _Word_ = "Hallo" };

and so on...

answered Nov 28 '14 at 16:17

Flat Eric

7,971
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Jon Skeet · Accepted Answer · 2014-11-28T16:31:03.433

The problem is that you're creating an array, but that array contains empty references to start with - you're never creating any instances of Word. You want something like:

Word[] Wörter = new Word[5];
Wörter[0] = new Word();
Wörter[0]._Word_ = "Hallo";
// etc

However, you can reduce the code to achieve the same effect using object initializers:

Word[] Wörter = new Word[5];
Wörter[0] = new Word { _Word_ = "Hallo" };
...

And then you can use an array initializer to do the whole thing in one statement:

Word[] Wörter =
{
    new Word { _Word = "Hallo" },
    new Word { _Word_ = "Flugzeug" },
    new Word { _Word_ = "Automobil" },
    new Word { _Word_ = "Musikanlage" },
    new Word { _Word_ = "Steuerung"}
};

The next step in my view would be to start following .NET naming conventions, calling your properties Text and Counter (you can't call the property Word as that's the name of the class), using automatically implemented properties to reduce cruft further, and then adding a constructor taking the initial value of the property in the constructor:

public class Word
{
    public string Text { get; set; }
    public int Counter { get; set; }

    public Word(string text)
    {
        Text = text;
    }
}

Word[] words =
{
    new Word("Hallo"),
    new Word("Flugzeug"),
    new Word("Automobil"),
    new Word("Musikanlage" )
    new Word("Steuerung")
};

Doesn't that look better? :)

You might also want to make the Text property read-only. Until C# 6 lands that means either keeping a private setter or using a "manually implemented" property, but in C# 6 you'll be able to write:

public string Text { get; }

and just assign it in the constructor.

Additionally, you need to think about whether you really need an array at all. Often using a List<T> offers significantly more flexibility.

Ah yes...damn sometimes I forget those obviously things. Thank your for the answer! :) — avbmath, Nov 28 '14 at 16:21
@avbmath: See all the rest of the answer for further suggestions. — Jon Skeet, Nov 28 '14 at 16:22

score 0 · Answer 3 · answered Nov 28 '14 at 16:20

0

Try this way:

   Word[] Wörter = new Word[5];

    for (int i = 0; i < 5; i++)
        Wörter[i] = new Word();

    Wörter[0]._Word_ = "Hallo";
    Wörter[1]._Word_ = "Flugzeug";
    Wörter[2]._Word_ = "Automobil";
    Wörter[3]._Word_ = "Musikanlage";
    Wörter[4]._Word_ = "Steuerung";

answered Nov 28 '14 at 16:20

Steve Wellens

20,506
2
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I think thats the best way for long arrays. Thank you! – avbmath Nov 28 '14 at 16:22
1

@avbmath: I don't think it's cleaner than using an array initializer... see the options in my answer. – Jon Skeet Nov 28 '14 at 16:23

NullReferenceException when assigning property of array item

3 Answers3