Selectedindex is 0/-1 in listbox

Question

public partial class Form2 : Form
{
    public Form2()
    {
        InitializeComponent();
        checkxml();
    }

    private void Form2_Load(object sender, EventArgs e)
    {

    }
    private void Form2_FormClosing(object sender, FormClosingEventArgs e)
    {
        Environment.Exit(0); 
    }

    private void displayMessageBox(object sender, FormClosingEventArgs e)
    {
    }


    private void sluitenToolStripMenuItem_Click(object sender, EventArgs e)
    {
        this.Close();
    }

    private void button1_Click(object sender, EventArgs e)
    {           
        Form3 form3 = new Form3();
        form3.Show();

    }

    private void button2_Click(object sender, EventArgs e)
    {
        XmlDocument xDoc = new XmlDocument();
        xDoc.Load("mp3spelers.xml");
        string text = listBox1.GetItemText(listBox1.SelectedItem);
        foreach (XmlNode xNode in xDoc.SelectNodes("Players/Player"))
        {
            if (xNode.SelectSingleNode("make").InnerText == text) xNode.ParentNode.RemoveChild(xNode);
            xDoc.Save("mp3spelers.xml");
        }
    }

     public  List<string> _items = new List<string>();

    public void checkxml()
    {

        string[] arr = XDocument.Load("mp3spelers.xml").Root
                       .Descendants("make")
                       .Select(make => (string)make)
                       .ToArray();
        for (int i = 0; i < arr.Length; i++)
        {
            _items.Add(arr[i]);
        }

        listBox1.DataSource = _items;
    }

    public int indexnumber;

    private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
    {          
        indexnumber = listBox1.SelectedIndex;
        loadListBox(indexnumber, 0, "load");                       
    }


    public void loadListBox(int index, int mutatie, string action)
    {
        List<string> _modle = new List<string>();
        string[] arr1 = XDocument.Load("mp3spelers.xml").Root
                      .Descendants("modle")
                      .Select(modle => (string)modle)
                      .ToArray();

        List<string> _mbstorage = new List<string>();
        string[] arr2 = XDocument.Load("mp3spelers.xml").Root
                      .Descendants("mbstorage")
                      .Select(mbstorage => (string)mbstorage)
                      .ToArray();

        List<string> _price = new List<string>();
        string[] arr3 = XDocument.Load("mp3spelers.xml").Root
                      .Descendants("price")
                      .Select(price => (string)price)
                      .ToArray();
        List<string> _storage = new List<string>();
        string[] arr4 = XDocument.Load("mp3spelers.xml").Root
                      .Descendants("storage")
                      .Select(storage => (string)storage)
                      .ToArray();


        switch (action)
        {
            case "load":                    
                textBox1.Clear();
                textBox1.Text = "Model: " + arr1[index] + "\r\n" + "Opslag in MB: " + arr2[index] + "\r\n" + "Prijs: " + arr3[index] + "\r\n" + "Voorraad: " + arr4[index];
                break;
            case "mutate":
                int indexnumber = listBox1.SelectedIndex;
                int mutatienummer = Convert.ToInt32(arr4[indexnumber]);
                int mutatienummer2 = mutatienummer + mutatie;

                            XmlDocument xDoc = new XmlDocument();
                            xDoc.Load("mp3spelers.xml");
                            string text = arr4[indexnumber];
        foreach (XmlNode xNode in xDoc.SelectNodes("Players/Player"))
        {
            if (xNode.SelectSingleNode("storage").InnerText == text)

            xDoc.Save("mp3spelers.xml");
        }


                break;
        }
    }

    private void infoToolStripMenuItem_Click(object sender, EventArgs e)
    {
        listBox1.Hide();
        button1.Hide();
        button2.Hide();
        button3.Hide();
        textBox1.Clear();
        textBox1.Text = "Soundsharp";
    }

    private void mP3SpelersToolStripMenuItem_Click(object sender, EventArgs e)
    {
        listBox1.Show();
        button1.Show();
        button2.Show();
        button3.Show();
    }

    private void button3_Click(object sender, EventArgs e)
    {
        Form4 form4 = new Form4();
        form4.Show();
    }

    private void button4_Click(object sender, EventArgs e)
    {
        listBox1.SelectedIndex = 0;
        listBox1.DataSource = null;
        checkxml();
        listBox1.SelectedIndex = 1;

    }
}
}

I get the error when I try to assign value to a textbox from the selectedindex but the selectedindex is 0/-1 and crashes the program so what I tried to do was assign a selectedindex on a refresh button(button 4) but doesn't seem to be working.

An exception of type 'System.IndexOutOfRangeException' occurred

Error happens on the case load when it try to assign a value to the textbox by getting the indexnumber off the listbox so it knows what is selected.

Answer 1

Step 1: if there are no items in the LisBox make selected index as -1

Replace This:

listBox1.SelectedIndex = 0;

With This:

listBox1.SelectedIndex = -1;

Step 2: while getting the selctedindex value first check if there are any selected items or not

Replace This:

private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
{          
    indexnumber = listBox1.SelectedIndex;
    loadListBox(indexnumber, 0, "load");                       
}

WIth This:

private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
{          
    if(listBox1.SelectedIndex >= 0)
    {
      indexnumber = listBox1.SelectedIndex;
      loadListBox(indexnumber, 0, "load");                       
    }
}

Answer 2

if selectedindex is 0, the first element is selected, if its -1, then no element is selected. You should only get an IndexOutOfRangeException if you try to access your arrays at position -1.

First step is to check if selected index is -1, and if, not try to access an array at that position.

Set a breakpoint at your selection changed event handler, and look if it gets called multiple times maybe.