CODEWITHSUNDEEP
c#listboxnullreferenceexception
Nigel.Harris
Nigel.Harris

Reputation: 77

C# ListBox Selected Item Null Exception

The user can click an item in a ListBox as follows:

private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    textBox2.Clear();
    listBox2.Items.Clear();

    string[] p = 
         Directory.GetFiles(
             textBoxDir.Text, 
             listBox1.SelectedItem.ToString(),
             SearchOption.AllDirectories);

    foreach (string open in p) 
        ......
}

All is fine. However if the user clicks on an empty space in the ListBox, it displays the following error:

System.NullReferenceException

This is because of this line of code:

string[] p = 
    Directory.GetFiles(
        textBoxDir.Text, 
        listBox1.SelectedItem.ToString(),   
        SearchOption.AllDirectories);

Does anyone have a clever work around? Or suggest an aalternative to my code?

Upvotes: 4

Views: 24625

Answers (7)

alejo capdevila
alejo capdevila

Reputation: 11

I had a similar problem. This is the shortest way i fixed it.

    private void button1_Click(object sender, EventArgs e)
    {
        radioButton1.Checked = !radioButton1.Checked;
        string indicadorPais = "Something";
        if (listaPaises.SelectedItem != null)
        {
            indicadorPais = listaPaises.SelectedItem.ToString();
            label1.Text = indicadorPais;

        }

As a sidenote. My condition originally was comparing listaPaises.SelectedItem.ToString() != null. This broke because i was casting null to a string. The code im sending works fine.

Upvotes: 0

devdigital
devdigital

Reputation: 34349

The workaround is to check for a null value, and exit early.

if (listBox1.SelectedItem == null)
{
    return;
}

This avoids the nesting introduced by the other answers, which makes the code less readable.

Upvotes: 13

fred conklin
fred conklin

Reputation: 11

However none of these allow you to go back in and re-select from the list box. Still working on that issue - will update.

while (user == null) {
    try {
        user = this.lstAdministratorName.SelectedItem.ToString();
    } catch { 
        lstAdministratorName.ClearSelected(); 
        return; 
    }
}

Upvotes: -1

lem.mallari
lem.mallari

Reputation: 1275

just check first if listbox1.SelectedItem is empty or not prior to calling this line:

string[] p = Directory.GetFiles(textBoxDir.Text, listBox1.SelectedItem.ToString(),   SearchOption.AllDirectories);

Upvotes: 1

Nikhil Agrawal
Nikhil Agrawal

Reputation: 48558

How about

if (listBox1.SelectedItem != null)
{
    string[] p = Directory.GetFiles(textBoxDir.Text, listBox1.SelectedItem.ToString(),   SearchOption.AllDirectories);
}

Full Code

private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    if (listBox1.SelectedItem != null)
    {    
        textBox2.Clear();
        listBox2.Items.Clear();

        string[] p = Directory.GetFiles(textBoxDir.Text, listBox1.SelectedItem.ToString(), SearchOption.AllDirectories);

        foreach (string open in p) 
         ...... }
    }
}

Upvotes: 1

Hossein Narimani Rad
Hossein Narimani Rad

Reputation: 32481

You can check SelectedIndex before that line:

if(listBox2.SelectedIndex < 0)
    return;

Upvotes: 4

mauris
mauris

Reputation: 43619

How about doing a

if(listBox1.SelectedItem != null){
    // ... do your work with listBox1.SelectedItem here
}

that should prevent that error from happening.

Upvotes: 1

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