Advanced Filter

 According to your description, you'd like to make the item list popped out when TextChanged, and also with the corresponding item been highlighted in ComboBox. 

private void comboBox1_TextChanged(object sender, EventArgs e)

{

comboBox1.SelectedIndex = comboBox1.FindStringExact(comboBox1.Text);

comboBox1.DroppedDown = true;

comboBox1.Select(comboBox1.Text.Length, 0);

}