In C# .Net 3.5 Windows Form application, implement type-ahead search of ListBox for typed text and set selected index to first match

You mentioned:

a ListBox’s DataSource utilizes a bounded DataTable filled from a database SQL query.

Accordingly, lbItems.Items[i].ToString() does not return what you think, the item's text, it returns System.Data.DataRowView which is the type of the item when the DataSource is a DataTable. You need to call the GetItemText method to get the DisplayMember.

private void txtSearch_TextChanged(object sender, EventArgs e)
{
    var txt = txtSearch.Text.Trim();

    for (int i = 0; i < lbItems.Items.Count; i++)
    {
        bool b = false;

        if (txt.Length > 0)
        {
            var itemText = lbItems.GetItemText(lbItems.Items[i]);
            b = itemText.IndexOf(txt, StringComparison.InvariantCultureIgnoreCase) >= 0;
        }
                
        lbItems.SetSelected(i, b);
    }
}

Alternatively and instead of selecting the matched items, you can filter the view by setting the DataTable.DefaultView.RowFilter property. You just need to write:

private void txtSearch_TextChanged(object sender, EventArgs e)
{
    var filter = txtSearch.Text.Trim().Length > 0
        ? $"{lbItems.DisplayMember} LIKE '%{txtSearch.Text}%'"
        : null;

    (lbItems.DataSource as DataTable).DefaultView.RowFilter = filter;
}

The first answer did not work for me as needed. The ListBox just scrolled for too long and no item became selected.

I went the indexing route in an attempt to get a working solution.

The ListBox DataSource binds to a DataTable filled by DataAdapter from a simple SQL database query of a company id and company name from a company table.

OleDbDataAdapter daList = new OleDbDataAdapter();

/*
SQL query (sSQL) of DataAdapter.
SELECT COMP_ID, COMP_ID||' - '||COMP_NAME COMP_NAME FROM COMP ORDER BY COMP_ID
*/
daList.SelectCommand = new OleDbCommand(sSQL, clsMain.conBLASR);

DataTable dtList = new DataTable();
daList.Fill(dtList);

if (dtList.Rows.Count == 0)
{
  lbItems.Enabled = false;
  lbItems.Items.Add("No data found");
  cmdOK.Enabled = false;
}
else
{
  //Bind the ListBox to the DataTable.
  lbItems.DataSource = dtList;          
  lbItems.DisplayMember = sDisplayName; // sDisplayName = "COMP_NAME"
  bItems.ValueMember = sKeyName;        // sKeyName = "COMP_ID"
  lbItems.SelectedValue = sKeyValue;    // sKeyValue = sCompId
}

The following code works for finding the text in the TextBox within the ListBox and selecting the matching item.

private void txtSearch_TextChanged(object sender, EventArgs e)
{
    if (!string.IsNullOrEmpty(txtSearch.Text))
    {
        for (int i = 0; i < lbItems.Items.Count - 1; i++)
        {
            DataRowView drvLBItem = (DataRowView)lbItems.Items[i];

            String lbString = drvLBItem["COMP_NAME"].ToString().ToUpper();

            if (lbString.Contains(txtSearch.Text.ToUpper()))
            {
                lbItems.SetSelected(i, true);
                break;
            }
        }
    }
}

I would like to not iterate through all the items and maybe use LINQ or some other technique to directly access the ListBox item that contains the TextBox's text and make it the selected item.

If you want to go the indexing route, here's a solution that lets you skip a lot of entries and jump based on the first letter typed. After the first letter, it's a sequential scan. You could refine it if you liked.

First, add a list to hold the words (that list will be bound to the Listbox as a data source) and a dictionary to hold the index:

private List<string> _wordsList = new List<string>();
private Dictionary<string, int> _wordsIndex = new Dictionary<string, int>();
private int _lastIndexFound = 0;

I scrapped a handful of web pages (including this one and some news stuff), copying and pasting to Notepad (to convert the clipboard to text only) and added the file to the project with "Copy Always". At startup, I read that in, throw away everything that isn't a word, and sort it. The result is a list with a couple of thousand unique words, in case-independent sort order:

var fileContents = File.ReadAllText("TextFile1.txt");
var words = fileContents.Split(' ', '\t', '\r', '\n');
const string wordPattern = @"^[a-zA-Z].*";
var wordRegex = new Regex(wordPattern);
foreach (var word in words)
{
    if (wordRegex.IsMatch(word))
    {
        _wordsList.Add(word);
    }
}
_wordsList = _wordsList.OrderBy(word => word).Distinct().ToList();

Then I build the index:

var curLetter = "a";
var lastLetter = "z";
var curIndex = 0;
_wordsIndex.Add(curLetter, curIndex);
foreach(var word in _wordsList)
{
    ++curIndex;
    if (!word[0].ToString().Equals(curLetter, StringComparison.OrdinalIgnoreCase))
    {
        var nextCurLetterChar = (char)(curLetter[0] + 1);
        curLetter = nextCurLetterChar.ToString();
        if (string.Compare(curLetter, lastLetter, true) <=0)
        {
            _wordsIndex.Add(curLetter, curIndex);
        }
    }
}

Finally, here's my TextEntered event code:

private void textBox1_TextChanged(object sender, EventArgs e)
{
    var enteredText = textBox1.Text;
    if (string.IsNullOrEmpty(enteredText))
    {
        listBox1.SelectedIndex = -1;
        _lastIndexFound = 0;
        return;
    }
    var firstLetter = (char)enteredText[0];
    if (_wordsIndex.TryGetValue(firstLetter.ToString(), out var firstLetterIndex))
    {
        for(var i = firstLetterIndex; i < _wordsList.Count; i++)
        {
            if (_wordsList[i].StartsWith(enteredText, true, null))
            {
                _lastIndexFound = i;
                break;
            }
        }
        listBox1.SelectedIndex = _lastIndexFound;
    }
}

It's pretty quick and dirty code and it's had only light testing. But, it seems to work.