WPF override dependency property metdata is not working

Question

I have a requirement to monitor the topmost propertyvalue change on WPF Window. I am writing something like this:

static MainWindow()
        {
           TopmostProperty.OverrideMetadata(typeof(Window), new PropertyMetadata(new PropertyChangedCallback(Changed)));
        }

        public MainWindow()
        {
            InitializeComponent();
        }

        private static void Changed(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            throw new NotImplementedException();
        }

But I am getting this exception: he invocation of the constructor on type 'WpfApplication4.MainWindow' that matches the specified binding constraints threw an exception.' Line number '4' and line position '9'."

You wrote code that explicitly throws an exception. Why are you surprised when it does exactly what you told it to? — SLaks, Sep 24 '14 at 20:51

score 5 · Accepted Answer · edited May 23 '17 at 11:45

5

Two mistakes:

The first argument of OverrideMetadata must be your type.

The type of the second argument must be the same as in the base type.

TopmostProperty.OverrideMetadata(
    typeof(MainWindow),
    new FrameworkPropertyMetadata(Changed));

(Bonus) You don't need to override metadata if you just need change notification.

edited May 23 '17 at 11:45

Community

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answered Sep 24 '14 at 20:54

Athari

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I didn't know about #2 - is it documented anywhere? – Adi Lester Sep 24 '14 at 20:58
2

@AdiLester See [Framework Property Metadata - Specifying Metadata](http://msdn.microsoft.com/en-us/library/ms751554.aspx#Specifying_Metadata): `For existing properties (AddOwner or OverrideMetadata call), you should always override with the metadata type used by the original registration`. – Clemens Sep 24 '14 at 21:01

WPF override dependency property metdata is not working

1 Answers1