WPF: Creating attached bindable property

Question

I'm trying to add a property, that can be attached to any control and bind a value to it.

public class ValidationBorder : DependencyObject
    {
        public static readonly DependencyProperty HasErrorProperty =
            DependencyProperty.Register(
                "HasError",
                typeof(bool?),
                typeof(UIElement),
                new PropertyMetadata(default(Boolean))
            );

        public bool? HasError
        {
            get { return (bool?) GetValue(HasErrorProperty); }
            set {  SetValue(HasErrorProperty, value);}
        }

        public static void SetHasError(UIElement element, Boolean value)
        {
            element.SetValue(HasErrorProperty, value);
        }
        public static Boolean GetHasError(UIElement element)
        {
            return (Boolean)element.GetValue(HasErrorProperty);
        }
    }

My usage:

<TextBox Text="{Binding SelectedFrequencyManual, UpdateSourceTrigger=PropertyChanged}" TextAlignment="Center"
                         attached:ValidationBorder.HasError="{Binding Path=DataOutOfRange}">
                </TextBox>

When I start the project it displays error (translated):

A Binding cannot be specified in the TextBox setHasError property. Only a Binding can be specified in a DependencyProperty of a DependencyObject

What can be wrong with it?

I've tried everything I could find on the web:

• Add parenthesis in the binding
• Add RelativeSource
• Change DependencyProperty.Register to DependencyProperty.RegisterAttached
• Different types for typeof(UIElement) including typeof(TextBox)

Answer 1

Try this implementation:

public class ValidationBorder
{
    public static readonly DependencyProperty HasErrorProperty =
        DependencyProperty.RegisterAttached(
            "HasError",
            typeof(bool?),
            typeof(ValidationBorder),
            new PropertyMetadata(default(bool?))
        );

    public static void SetHasError(UIElement element, bool? value)
    {
        element.SetValue(HasErrorProperty, value);
    }
    public static bool? GetHasError(UIElement element)
    {
        return (bool?)element.GetValue(HasErrorProperty);
    }
}

You should call RegisterAttached and set the owner type to ValidationBorder. Also remove the HasError property. Please refer to the docs for more information.