Custom Dependency Properties and TwoWay binding in WPF

Question

We have an object that derives from DependencyObject, and implements some DependencyProperties.

Basically something like this:

class Context : DependencyObject {
   public static readonly DependencyProperty NameProperty =
   DependencyProperty.Register ("Name", typeof (string), typeof (Context), new PropertyMetadata (""));
    public string Name {
        get {
            return (string)this.GetValue (NameProperty);
        }
        set {
            this.SetValue (NameProperty, value);
        }
    }
}

This works, the property is setup, can be bound, etc. The issue comes when I bind TO the propery from WPF, using a TwoWay bind. The TwoWay part never actually happens, WPF never calls the set of this property. I have set my binding up like this:

<TextBox Text="{Binding Path=Name, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />

In this case, typing in the text box should immediately update the Name property, but it does not. If I change the Name property to be a regular POCO property, it works (though the other side of the TwoWay obviously doesn't unless I implement INotifyPropertyChanged).

What am I doing wrong here? This should be a really simple thing to do, but it's causing me no end of headaches.

score 11 · Accepted Answer · edited Sep 27 '11 at 15:12

11

This is expected behavior. The CLR property is merely a wrapper around the underlying DependencyProperty. WPF often optimizes by calling GetValue and SetValue directly. If you need custom logic to execute then use the metadata of the DependencyProperty.

edited Sep 27 '11 at 15:12

answered Apr 29 '09 at 17:29

Kent Boogaart

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Geez, I'm an idiot. You are 100% correct, the value itself is set correctly, but the setter is never called, which is what through me off. Thanks. – Matt Holmes Apr 29 '09 at 17:31

score 2 · Answer 2 · answered Jun 29 '11 at 08:00

After this issue cost me some time:

For those of you who have the same problem, but - like me - don't see the solution in the answer above:

To support inheritance of the DataContext, the custom class (Context in this case) has to be derived from FrameworkElement rather than DependencyObject. That's all.

Marc

Custom Dependency Properties and TwoWay binding in WPF

2 Answers2