Question

I have a textbox that is bound to a property that requires a value, ie:

 [Required(ErrorMessage = "required value")]
 public string SomeText
 {
     //get set...
 }

And in my XAML, I have the following setup for my textbox:

 UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=true, ValidatesOnExceptions=true

As expected, the red border appears when there is no value in the textbox, however when I select a different tab and then go back to the page with the invalid results, the red border no longer appears. It only reappears if I enter a valid result and then erase it.

How can I debug this? How can I find out what event causes the red border to appear?

Answer 1

In WPF when items on the tab get unloaded from the visual tree the fact that they were marked invalid is lost. Basically, when a validation error happens the UI responds to an event in the validation stack and marks the item invalid. This marking doesn’t get re-evaluated when the item comes back into the visual tree unless the binding is also re-evaluated (which it usually isn’t if the user clicks on a tab item).

Define a function like this somewhere (I put it in a static ValidationHelper class along with some other things):

public static void ReMarkInvalid( DependencyObject obj )
{
    if( Validation.GetHasError( obj ) ) {
        List<ValidationError> errors = new List<ValidationError>( Validation.GetErrors( obj ) );
        foreach( ValidationError error in errors ) {
            Validation.ClearInvalid((BindingExpressionBase)error.BindingInError);
            Validation.MarkInvalid((BindingExpressionBase)error.BindingInError, error);
        }
    }

    for( int i = 0; i < VisualTreeHelper.GetChildrenCount( obj ); i++ ) {
        ReMarkInvalid( VisualTreeHelper.GetChild( obj, i ) );
    }
}

I think you can call this function in the TabControl’s Selected event and it should have the desired effect. E.g.:

private void TabControl_Selected(...) 
{
    ReMarkInvalid( tabControl );
}

If that doesn’t work you may need to do this at a lower Dispatcher priority to make sure the visual tree has finished loading first. Which would look like replacing ReMarkInvalid… with:

Dispatcher.BeginInvoke( new Action( delegate()
{
    ReMarkInvalid( tabControl );
} ), DispatcherPriority.Render );