Why can't get validation error display in validationSummary?

Question

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<ValidationErrorEventArgs>((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?

Answer 1

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
}