TextBox with validation loses ErrorTemplate on tab change

Question

I have a TextBox with a validation rule that is on a tab of a TabControl. The default ErrorTemplate correctly shows (red border around TextBox) when the validation rule fail

Answer 1

As Dylan explained, this is because the Adorner layer, in which the validation errors are drawn, is discarded on tab switch. So you need to wrap the content with AdornerDecorator.

I have created a behavior that wraps the Content of TabItem automatically in an AdornerDecorator, so that it doesn't have to be done manually on all TabItems.

public static class AdornerBehavior
{
    public static bool GetWrapWithAdornerDecorator(TabItem tabItem)
    {
        return (bool)tabItem.GetValue(WrapWithAdornerDecoratorProperty);
    }
    public static void SetWrapWithAdornerDecorator(TabItem tabItem, bool value)
    {
        tabItem.SetValue(WrapWithAdornerDecoratorProperty, value);
    }

    // Using a DependencyProperty as the backing store for WrapWithAdornerDecorator.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty WrapWithAdornerDecoratorProperty =
        DependencyProperty.RegisterAttached("WrapWithAdornerDecorator", typeof(bool), typeof(AdornerBehavior), new UIPropertyMetadata(false, OnWrapWithAdornerDecoratorChanged));

    public static void OnWrapWithAdornerDecoratorChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
    {
        var tabItem = o as TabItem;
        if (tabItem == null) return;

        if(e.NewValue as bool? == true)
        {
            if (tabItem.Content is AdornerDecorator) return;
            var content = tabItem.Content as UIElement;
            tabItem.Content = null;
            tabItem.Content = new AdornerDecorator { Child = content };
        }
        if(e.NewValue as bool? == false)
        {
            if (tabItem.Content is AdornerDecorator)
            {
                var decorator= tabItem.Content as AdornerDecorator;
                var content = decorator.Child;
                decorator.Child = null;
                tabItem.Content = content;
            }
        }
    }
}

You can set this behavior on all TabItems via a default style:

b is the namespace where the behavior is located, something like this (will be different for every project):

xmlns:b="clr-namespace:Styling.Behaviors;assembly=Styling"