Search in A ListBox

问题:

HI

I'm trying to put a textbox to search in a listBox. I have a TextBox: SearchText with this code:

 private void SearchText_TextChanged(object sender, EventArgs e)
 {
    int i = listBox3.FindString(SearchText.Text);
    listBox3.SelectedIndex = i;
 }

and a ListBox On the Load I have this code

List<string> str = GetListOfFiles(@"D:\\Music\\massive attack - collected");        
listBox3.DataSource = str;
listBox3.DisplayMember = "str";

and on selectedIndexChanged :

private void listBox3_SelectedIndexChanged(object sender, EventArgs e)
{
    player1.URL = listBox3.SelectedItem.ToString(); // HERE APPEAR THE ERROR "Object reference not set to an instance of an object."

    //  provaTxt.Text = listBox3.SelectedValue.ToString();
}

When I write down in the SeachText to find a songs I receive an error ("Object reference not set to an instance of an object.") in the line selectedIndexChanged of the ListBox. Do you know one more way to find in a listBox as my case? Thanks for your share. Nice Regards

回答1:

It sounds like the item wasn't found, so SelectedItem was null; try using:

player1.URL = Convert.ToString(listBox3.SelectedItem);

I believe this handles the null case (altenatively, test for null first).

I'd also be tempted to look in the underlying list:

List<string> items = (List<string>)listbox3.DataSource;
listbox3.SelectedIndex = items.FindIndex(s => s.StartsWith(searchFor));

---

For example:

using System;
using System.Collections.Generic;
using System.Windows.Forms;
class MyForm : Form
{
    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        Application.Run(new MyForm());
    }

    ListBox listbox;
    TextBox textbox;
    CheckBox multi;
    public MyForm()
    {
        textbox = new TextBox { Dock = DockStyle.Top };
        List<string> strings = new List<string> { "abc", "abd", "abed", "ab" };
        listbox = new ListBox { Dock = DockStyle.Fill, DataSource = strings };
        textbox.KeyDown += textbox_KeyDown;
        Controls.Add(listbox);
        Controls.Add(textbox);
        listbox.SelectedIndexChanged += listbox_SelectedIndexChanged;
        listbox.SelectionMode = SelectionMode.MultiExtended;
        multi = new CheckBox { Text = "select multiple", Dock = DockStyle.Bottom };
        Controls.Add(multi);
    }

    void listbox_SelectedIndexChanged(object sender, EventArgs e)
    {
        Text = Convert.ToString(listbox.SelectedItem);
    }

    void textbox_KeyDown(object sender, KeyEventArgs e)
    {
        if (e.KeyCode == Keys.Return)
        {
            string searchFor = textbox.Text;
            List<string> strings = (List<string>)listbox.DataSource;
            if (multi.Checked)
            {
                for (int i = 0; i < strings.Count; i++)
                {
                    listbox.SetSelected(i, strings[i].Contains(searchFor));
                }
            }
            else
            {
                listbox.ClearSelected();
                listbox.SelectedIndex = strings.FindIndex(
                    s => s.Contains(searchFor));
            }
        }
    }
}