title: ListBox SelectionChanged event : get the value before it was changed
body: <p>I'm working on a C# wpf application in which there is a listbox, and I'd like to get the value of the element that was selected before a change occur</p>
  
  <p>I succeeded in getting the new value this way :</p>
  
  <pre><code>&lt;ListBox SelectionChanged="listBox1_SelectedIndexChanged"... /&gt;
  
  private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
      {
          test.add(listBox1.SelectedItem.ToString());
      }
  </code></pre>
  
  <p>But I would need something like <code>listBox1.UnselectedItem</code> to get the element that was unselected during the change. Any idea ?</p>