title: asp.net, validate as user is writing on textbox
body: <p>I'm working on ASP.NET with C#.</p>
  
  <p>I have a <code>TextBox</code> and I want to validate if the user is writing the correct length for the <code>TextBox</code>.</p>
  
  <p>If the user is not entering the correct length I want a message to display on a label. But I want this check to happen on the fly, as the user is writing.</p>
  
  <p>How can I do this? </p>
  
  <p>Thanks.</p>