Even though the xaml interpreter happens to turn the markup extension into something working, this is not really supported.
See MSDN – Binding Markup Extension
The following are properties of Binding that cannot be set using the Binding markup extension/{Binding} expression form.
…
ValidationRules: the property takes a generic collection of ValidationRule objects. This could be expressed as a property element in a Binding object element, but has no readily available attribute-parsing technique for usage in a Binding expression. See reference topic for ValidationRules.
However, let me suggest a different approach: instead of nesting the custom markup extension in the binding, nest the binding in a custom markup extension:
[ContentProperty("Binding")]
[MarkupExtensionReturnType(typeof(object))]
public class BindingEnhancementMarkup : MarkupExtension
{
public BindingEnhancementMarkup()
{
}
public BindingEnhancementMarkup(Binding binding)
{
Binding = binding;
}
[ConstructorArgument("binding")]
public Binding Binding { get; set; }
public override object ProvideValue(IServiceProvider serviceProvider)
{
Binding.ValidationRules.Add(new NotEmptyValidationRule());
return Binding.ProvideValue(serviceProvider);
}
}
And use as follows:
<TextBox Text="{local:BindingEnhancementMarkup {Binding Path=Location, UpdateSourceTrigger=PropertyChanged}}"/>
Ofcourse, for production you may want to add a few more checks in the markup extension instead of just assuming everything is in place.