title: MVC2 model validation mutually exclusive required annotation
body: <p>Does anybody know of a good algorithm to mutually exclusively check two properties using a <code>ModelValidator</code>?</p>
  
  <p>Something like:</p>
  
  <pre><code>    [EitherPropertyRequired("BuildingNumber","BuildingName"]
      public class Address{
         public int BuildingNumber { get; set; }
         public string BuildingName { get; set; }
     }
  </code></pre>