Why can't get validation error display in validationSummary?

I have a form with some validations set in entity metadata class. and then binding entity instance to UI by VM. Something as below:

Xaml like:

   <Grid x:Name="LayoutRoot">
            <StackPanel VerticalAlignment="Top">
                <input:ValidationSummary />
            </StackPanel>
      <TextBox Text="{Binding Name, Mode=TwoWay}" />
      <ComboBox x:Name="xTest" ItemsSource="{Binding MyList}"
               SelectedItem="{Binding MyItem,Mode=TwoWay,
               DisplayMemberPath="MyName"
               ValidatesOnDataErrors=True,
               ValidatesOnNotifyDataErrors=True,
               ValidatesOnExceptions=True,
               NotifyOnValidationError=True,UpdateSourceTrigger=Explicit}"  />
      </Grid>

Code-behind like:

public MyForm()
{
  InitializeComponent();
  this.xTest.BindingValidationError +=new EventHandler<ValidationErrorEve开发者_C百科ntArgs>((s,e)=>{

  BindingExpression be = this.xTest.GetBindingExpression(ComboBox.SelectedItemProperty);
  be.UpdateSource();

  if (e.Action == ValidationErrorEventAction.Added)
    ((ComboBox)s).Foreground = new SolidColorBrush(Colors.Red);
  });
}

Metadata like:

[Required]
 public string Name { get; set; }

 [RequiredAttribute]
 public int MyItemID { get; set; }

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?

Why are you using an Explicit UpdateSourceTrigger?

Silverlight validation happens inside the binding framework, when the binding is updating the source object. The way you have this, there won't be a binding validation error because you never tell the binding to update the source object. Well, actually you do, but it happens inside the validation error event handler. You've written chicken-and-egg code.

• Remove your UpdateSourceTrigger on your binding or set it to Default.
• Remove the explicit call to BindingExpression.UpdateSource.
• Remove setting the ComboBox foreground to red - you are using NotifyOnValidationError=True, which eliminates any need to manually color the control.
• Remove the DisplayMemberPath from the binding

So your XAML:

<Grid x:Name="LayoutRoot"> 
    <StackPanel VerticalAlignment="Top"> 
         <input:ValidationSummary /> 
         <ComboBox x:Name="xTest" ItemsSource="{Binding MyList}" 
               SelectedItem="{Binding MyItem,
               Mode=TwoWay, 
               ValidatesOnDataErrors=True, 
               ValidatesOnNotifyDataErrors=True, 
               ValidatesOnExceptions=True, 
               NotifyOnValidationError=True}"  /> 
      </StackPanel> 
</Grid> 

And your code:

public MyForm()   
{   
  InitializeComponent();   
  // you don't need anything here to have the validations work
}