title: Allow only alphanumeric in textbox
body: <p>I have a textbox that should disallow entering any special characters.</p>
  
  <p>The user can enter :   </p>
  
  <ol>
  <li>A-Z  </li>
  <li>a-z  </li>
  <li>0-9  </li>
  <li>Space</li>
  </ol>
  
  <p>How can I make the <code>KeyDown</code> event to do this? </p>