title: Why can't get validation error display in validationSummary?
body: <p>I have a form with some validations set in entity metadata class. and then binding entity instance to UI by VM. Something as below:</p>
  
  <p>Xaml like:</p>
  
  <pre><code>   &lt;Grid x:Name="LayoutRoot"&gt;
              &lt;StackPanel VerticalAlignment="Top"&gt;
                  &lt;input:ValidationSummary /&gt;
              &lt;/StackPanel&gt;
        &lt;TextBox Text="{Binding Name, Mode=TwoWay}" /&gt;
        &lt;ComboBox x:Name="xTest" ItemsSource="{Binding MyList}"
                 SelectedItem="{Binding MyItem,Mode=TwoWay,
                 DisplayMemberPath="MyName"
                 ValidatesOnDataErrors=True,
                 ValidatesOnNotifyDataErrors=True,
                 ValidatesOnExceptions=True,
                 NotifyOnValidationError=True,UpdateSourceTrigger=Explicit}"  /&gt;
        &lt;/Grid&gt;
  </code></pre>
  
  <p>Code-behind like:</p>
  
  <pre><code>public MyForm()
  {
    InitializeComponent();
    this.xTest.BindingValidationError +=new EventHandler&lt;ValidationErrorEventArgs&gt;((s,e)=&gt;{
  
    BindingExpression be = this.xTest.GetBindingExpression(ComboBox.SelectedItemProperty);
    be.UpdateSource();
  
    if (e.Action == ValidationErrorEventAction.Added)
      ((ComboBox)s).Foreground = new SolidColorBrush(Colors.Red);
    });
  }
  </code></pre>
  
  <p>Metadata like:</p>
  
  <pre><code>[Required]
   public string Name { get; set; }
  
   [RequiredAttribute]
   public int MyItemID { get; set; }
  </code></pre>
  
  <p>But when running the app, I got nothing display in valudationSummary.
  For CombBox, even there is error, looks like BindingValidationError event is never fired. 
  How to resolve it?</p>