title: Cancelling a WPF TextBox Changed Event
body: <p>I'm in the process of learning WPF coming from WinForms development.</p>
  
  <p>I have a TextChanged event assigned to one of my TextBox's in my WPF application.  If the user enters invalid data, I want to be able to revert to the previous text value.</p>
  
  <p>In the old forms day, I would replace NewValue with OldValue, but it seems WPF doesn't work the same way.</p>
  
  <p>Any ideas on what I could do it achieve this?  Am I just not thinking with WPF yet?</p>
  
  <p>Thanks.</p>