C# Arrays and Lists

Question

I want to add the items from the list box into a string array

However, when I run the code I get a Null Reference Exception on the variable declaration. Any ideas?

List items; string[] array;

    public MainWindow()
    {
        InitializeComponent();

        items = new List<String>();
        listBox.Items.Add("Kevin");
        listBox.Items.Add("James");
        listBox.Items.Add("John");
        listBox.Items.Add("Mathew");
        listBox.Items.Add("George");

Far easier to use C#'s rich data types. List, Dictionary and SortedList etc. — Lee Taylor, Apr 08 '13 at 02:38
What method calls ExchangeSort()? did you try to debug the code ? according to your error the array input parameter is null thus it is not sent by the calling method. — Mortalus, Apr 08 '13 at 02:39
How is the method being called? Looks like a null value is being passed in. — Arian Motamedi, Apr 08 '13 at 02:40
This is for homework. It's taken me 2 days and I can't seem to figure it out. — Daniel Xi, Apr 08 '13 at 02:43
The most possible source of problem is the assignments to `array`. — Alvin Wong, Apr 08 '13 at 02:44
how do you populate the array, what is its size? can i see the declaration also? — jacob aloysious, Apr 08 '13 at 02:49
In you button click event (Button_Click_2) you pass array to ExchangeSort but array is not defined in this function. Where is it defined? — Abe Miessler, Apr 08 '13 at 02:52
You asked this [question](http://stackoverflow.com/questions/15869351/modify-sorting-algorithm-to-work-with-listboxes/15869652#15869652) just before. The answer is the same for this: you could use LINQ. Else this seems like homework... — devilfish17, Apr 08 '13 at 03:24
@devilfish17 It is. I'm just trying to understand it when it comes to dealing with strings. Sorting it using integers I'm fine with but sorting the listbox with strings I'm having difficulty with. — Daniel Xi, Apr 08 '13 at 03:28

Matt · Answer 1 · 2013-04-08T03:55:53.697

0

You need to initialise the array, you have only declared it as a member of the class:

private void Button_Click_2(object sender, RoutedEventArgs e)
{
    array = new string[listbox.Items.Count];

    ExchangeSort(array);
    listBox.Items.Clear();
    foreach (string i in array)
    {
        listBox.Items.Add(i);
    }
    MessageBox.Show("Done");

}

edited Apr 08 '13 at 03:55

answered Apr 08 '13 at 03:32

Matt

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I get the error message: Error 1 'System.Windows.Controls.ItemCollection' does not contain a definition for 'Length' and no extension method 'Length' accepting a first argument of type 'System.Windows.Controls.ItemCollection' could be found (are you missing a using directive or an assembly reference?) – Daniel Xi Apr 08 '13 at 03:41
Sorry, it's Count, not Length – Matt Apr 08 '13 at 03:56

C# Arrays and Lists

1 Answers1