c# - Override Winforms ComboBox Autocomplete Suggest Rule

Question

I'm trying to modify the behaviour of a Windows.Forms ComboBox so that the AutoComplete drop down displays items according to the rules I specify.

By default, if you use AutoComplete in a ComboBox, the rule that's followed is "string s is included in the drop down if( s.StartsWith( userEnteredTextInTheComboBox) )" All I'm really interested in is substituting a new rule for the current one, but I can find no way to get at it. (Specifically, I'd prefer s.Contains instead of s.StartsWith.)

I can kludge together a clumsy solution using two controls instead of one, but I'd really be happier with one that actually does what I want.

Update: I found essentially the same question after some more searching. The answer supplied there suggests that using two controls to "fake it" is the way to go.

Answer 1

I've had the same problem and looked for a quick solution.

Eventually I ended up writing it myself. It's a little dirty but it should not be hard to make it prettier if needed.

The idea is to re-build the combo list after every key press. This way we can rely on the combo's built-in interface, and we don't need to implement our own interface with a textbox and a listbox...

Just remember to set combo.Tag to null if you re-build the combo's options list.

private void combo_KeyPress(object sender, KeyPressEventArgs e) {
    comboKeyPressed();
}

private void combo_TextChanged(object sender, EventArgs e) {
    if (combo.Text.Length == 0) comboKeyPressed();
}

private void comboKeyPressed() {
    combo.DroppedDown = true;

    object[] originalList = (object[])combo.Tag;
    if (originalList == null) {
        // backup original list
        originalList = new object[combo.Items.Count];
        combo.Items.CopyTo(originalList, 0);
        combo.Tag = originalList;
    }

    // prepare list of matching items
    string s = combo.Text.ToLower();
    IEnumerable<object> newList = originalList;
    if (s.Length > 0) {
        newList = originalList.Where(item => item.ToString().ToLower().Contains(s));
    }

    // clear list (loop through it, otherwise the cursor would move to the beginning of the textbox...)
    while (combo.Items.Count > 0) {
        combo.Items.RemoveAt(0);
    }

    // re-set list
    combo.Items.AddRange(newList.ToArray());
}