title: Determine when and which character is added or deleted in a Text Box
body: <p>I have a simple text box in a WPF application.</p>
  
  <p>I need to know <strong>when</strong> a character was added/deleted in the text box, <strong>which</strong> character and <strong>where</strong> it was added or deleted. </p>
  
  <p>I thought about using the <code>TextBox.KeyDown</code> event, but it has some problems:</p>
  
  <ul>
  <li>I can't know where the character was added or deleted.</li>
  <li>I have no idea how to determine which character was added (from the <code>KeyEventArgs</code>).</li>
  </ul>
  
  <p>Any ideas? </p>