كيفية عمل listbox تبادلي ؟؟؟

look this code

  protected void butAddWorkFields_Click(object sender, EventArgs e)
    {
        if (listWorkFieldsMain.SelectedIndex.ToString() == "-1") return;

        listWorkFieldsSelect.Items.Add(new ListItem(listWorkFieldsMain.SelectedItem.Text.ToString(), listWorkFieldsMain.SelectedItem.Value.ToString()));
        listWorkFieldsSelect.DataBind();

        listWorkFieldsMain.Items.RemoveAt(listWorkFieldsMain.SelectedIndex);
        listWorkFieldsMain.DataBind();
    }
    protected void butRemoveWorkFields_Click(object sender, EventArgs e)
    {
        if (listWorkFieldsSelect.SelectedIndex.ToString() == "-1") return;

        listWorkFieldsMain.Items.Add(new ListItem(listWorkFieldsSelect.SelectedItem.Text.ToString(), listWorkFieldsSelect.SelectedItem.Value.ToString()));
        listWorkFieldsMain.DataBind();

        listWorkFieldsSelect.Items.RemoveAt(listWorkFieldsSelect.SelectedIndex);
        listWorkFieldsSelect.DataBind();
    }