title: Extending sealed class
body: <p>I'm developing Windows Store application using javascript. I also have WinRT library for client-server interactions. Client receives from server a custom class <code>User</code>, which is sealed, as it's required for classes in WinRT lib. I need to extend <code>User</code> object for further use with data binding.</p>
  
  <p>What are best practices for doing it? Is there best way than creating new <code>UserDataBinding</code> class and copying all properties from <code>User</code>? </p>