title: ExpressiveAnnotations.Attributes RequiredIf validation not working
body: <p>I have the following validation not working and cannot resolve why that is the case?</p>
  
  <pre><code>    [Display(Name = "Bus")]
      [RequiredIf("SelectedWidgetText == 'Referral'", ErrorMessage = "Please select a Vehicle.")]
      public int DepotId{ get; set; }
  
      public string SelectedWidgetText { get; set; }
  </code></pre>
  
  <p>I have also tried the following:</p>
  
  <pre><code>[Display(Name = "Bus")]
  [AssertThat("SelectedWidgetText == 'Referral'", ErrorMessage = "Please select a Vehicle.")]
  public int DepotId{ get; set; }
  </code></pre>
  
  <p>on the server side, when the modelstate.isvalid method is hit, it returns false and the error message thrown is <code>The DepotId field is required.</code></p>