title: Restricting characters in a TextBox
body: <p>I'm building a form in a C# WinRT app, and I'd like to restrict the characters in one of the TextBox components to numerals only. (This TextBox would be for a user to enter a year into.)</p>
  
  <p>I've searched for a while, but haven't been able to figure this one out without setting up an event listener on the <code>TextChanged</code> event, and inspecting the <code>text</code> property on every key press. Is there a way to simply say that a user can only enter specific characters into a TextBox?</p>