ComboBox open while another control has focus

Question

I have a case where the user is given a ComboBox with potentially a lot of choices in it. Paired with this is a TextBox that filters the items. What I would like to do is open the drop down list when the TextBox has focus--let the user see what the current filter accomplishes as they type it. (This isn't just autocomplete, I'm currently matching the filter text anywhere in the item, I may replace this with a RegEx search down the road.)

It sounds simple enough--drop the box when the TextBox gets focus, close it when it loses focus. It opens--and promptly closes back up. Any good answers?

My Google-Fu must be weak tonight, I can't believe nobody has wanted to do this before yet I find nothing out there. (I have seen a related thing of typing in an open ComboBox to provide suggested options like Google does but my list is required, not merely suggestions.)

Answer 1

You can add on the Focus event of the TextBox code for the ComboBox setting the property

ComboBox.DroppedDown = true;

Than add on the TextChanged event of the TextBox code

ComboBox.SuspentLayout();
//ComboBox.Items add/remove
ComboBox.ResumeLayout();

Don't forget to reset the items when Text is empty.

EDIT:

This seems to work (but you don't get to see the mouse)

string[] items = { "abcd", "abc", "bcd", "cd" };

private void textBox1_TextChanged(object sender, EventArgs e)
{
    comboBox1.SuspendLayout();
    comboBox1.Items.Clear();
    comboBox1.Items.AddRange(items.Where(item => item.ToLower().Contains(textBox1.Text.ToLower())).ToArray());
    comboBox1.ResumeLayout();
    comboBox1.DroppedDown = true;
}