Question

I’m trying to bind a data value to an attached property. However, it just dont get it to work.

I defined it like:

public static class MyClass
{
    public static readonly DependencyProperty MyPropertyProperty =
        DependencyProperty.RegisterAttached("MyProperty", typeof(string),
        typeof(MyClass), new PropertyMetadata(null));

    public static string GetMyProperty(DependencyObject d)
    {
        return (string)d.GetValue(MyPropertyProperty);
    }

    public static void SetMyProperty(DependencyObject d, string value)
    {
        d.SetValue(MyPropertyProperty, value);
    }
}

Now the XAML I use it looks something like this:

<TextBlock local:MyClass.MyProperty="{Binding MyStringValue}" />

I set a breakpoint in the SetMyProperty method, but it is never called. It does not raise any error, it is just never set or asked for. However, if I change the value in XAML to a fixed string, it gets called:

<TextBlock local:MyClass.MyProperty="foobar" />

What am I missing?

Note: the above example is the minimal version that shows the same strange behavior. My actual implementation makes more sense than this of course.

Thanks in advance for any hint!

Answer 1

And the binding wont trigger your SetMyProperty ever – if you need controll over when the value changes you must use the override of PropertyMetadata that expects a “Change”-Handler

... new PropertyMetadata(
    null,
    new PropertyChangedCallback((sender, e) => {
      var myThis = (MyClass)sender;
      var changedString = (string)e.NewValue;
      // To whatever you like with myThis ( = the sender object) and changedString (= new value)
    })