title: Removing more than 1 item in listbox
body: <p>please help me how can remove more than 1 item in a <code>listbox</code>.I know the code for removing 1 item: <code>listbox.Items.RemoveAt(i)</code>
  but for more than 1 item selected in a listbox,what is the code? write the code for example in button-click event.I have only a button and a listbox in my winform.(write codes in C#) </p>