wpf combobox filter as you type

private void comboBox_KeyUp(object sender, KeyEventArgs e)
{
    var combobox = (ComboBox)sender;
    var ctb = combobox.Template.FindName("PART_EditableTextBox", combobox) as TextBox;
    if (ctb == null) return;
    if (Keyboard.Modifiers.HasFlag(ModifierKeys.Shift) || Keyboard.Modifiers.HasFlag(ModifierKeys.Control) || Keyboard.Modifiers.HasFlag(ModifierKeys.Alt))
    return;
    var caretPos = ctb.CaretIndex;
    combobox.IsDropDownOpen = true;

    CollectionView itemsViewOriginal = (CollectionView)CollectionViewSource.GetDefaultView(combobox.Items);
    itemsViewOriginal.Filter = ((o) =>
    {
        if (String.IsNullOrEmpty(combobox.Text))
        {
            return true;
        }
        else
        {
            if (((ComboBoxItem)o).Content.ToString().StartsWith(combobox.Text, true, null))
            {
                return true;
            }
            else
            {
                return false;
            }
        }
    });

    itemsViewOriginal.Refresh();
    ctb.CaretIndex = caretPos;
}