Why default value of attached property is not shown?

Question

I want a TextBlock to show the default value of an attached property, but it doesn't work.

XAML

<TextBlock Text="{Binding RelativeSource={RelativeSource Self}, Path=MyData}" />

Attached Property

public class MyDependencyObject
{
    public static readonly DependencyProperty MyDataProperty =
        DependencyProperty.RegisterAttached("MyData", typeof(string), typeof(TextBlock),
            new FrameworkPropertyMetadata("MyDependencyObject"));


    public static string GetMyData(DependencyObject dpo)
    {
        return (string)dpo.GetValue(MyDataProperty);
    }

    public static void SetMyData(DependencyObject dpo, string value)
    {
        dpo.SetValue(MyDataProperty, value);
    }
}

When I set MyData's value, it works well.

<TextBlock local:MyDependencyObject.MyData="Test"
           Text="{Binding RelativeSource={RelativeSource Self}, Path=MyData}" />

Answer 1

Data binding to an attached property requires a Path in parentheses, see PropertyPath for Objects in Data Binding:

<TextBlock Text="{Binding RelativeSource={RelativeSource Self},
                          Path=(local:MyDependencyObject.MyData)}"/>

The attached property declaration must use the declaring class as ownerType argument to RegisterAttached

public class MyDependencyObject
{
    public static readonly DependencyProperty MyDataProperty =
        DependencyProperty.RegisterAttached(
            "MyData",
            typeof(string),
            typeof(MyDependencyObject), // here
            new FrameworkPropertyMetadata("MyDependencyObject"));

    public static string GetMyData(DependencyObject obj)
    {
        return (string)obj.GetValue(MyDataProperty);
    }

    public static void SetMyData(DependencyObject obj, string value)
    {
        obj.SetValue(MyDataProperty, value);
    }
}

Although the attached property was never explicitly set on the TextBlock, it will show the "MyDependencyObject" default value.