title: IDataErrorInfo/WPF Binding - Validation over many ViewModels
body: <p>I'm trying to find an easy way to validate over a collection of ViewModels, using the IDataErrorInfo Interface.</p>
  
  <p>I have a ListBox, which is bound to an ObservableCollection of ViewModels.</p>
  
  <ul>
  <li>1 Class "<code>DataView&lt;VMUser&gt;</code>" with an <code>ObservableCollection&lt;VMUser&gt;</code> </li>
  <li>1 ViewModel Class "<code>VMUser</code>"</li>
  </ul>
  
  <p>If I implement the IDataErrorInfo into my ViewModel, i can validate for example if the Age > 21 and so on... But i cannot validate if there is no other user with the same email for example, because the ViewModels don't know anything from each other.</p>
  
  <p>I didn't find a way to force the Bindings in my VMUser-DataTemplate to use the IDataErrorInfo of the DataView Class. (without clicking the OK-Button...)</p>