Question

In my WPF project I need to animate several properties with the same value. So my idea was to create a custom, private dependency property to which the animation will be applied. Unfortunately this doesn’t seem to work. DependencyPropertyDescriptor.FromProperty() always returns null for this property. Here is the code:

public partial class PedestrianVisual : UserControl {
  private static readonly DependencyProperty CurrentInaccuracyRadiusProperty =
    DependencyProperty.Register("CurrentInaccuracyRadius", typeof(double), typeof(PedestrianVisual));
  private double CurrentInaccuracyRadius {
    get { return (double)GetValue(CurrentInaccuracyRadiusProperty); }
    set { SetValue(CurrentInaccuracyRadiusProperty, value); }
  }

  public PedestrianVisual() {
    InitializeComponent();

    // This returns "null" all the time.
    DependencyPropertyDescriptor dpd = DependencyPropertyDescriptor.FromProperty(
      CurrentInaccuracyRadiusProperty, typeof(PedestrianVisual));
    dpd.AddValueChanged(this, (s, e) => { 
      UpdateInaccuracyCircle((double)GetValue(CurrentInaccuracyRadiusProperty)); 
    });
  }

  private void UpdateInaccuracyCircle(double curRadius) {
    // do something here
  }
}

Is there any other way to create a private dependency property?

Answer 1

I don’t understand why you would do it that way, i did not encounter any problems when attaching the callback in the declaration, e.g. something like this:

private static readonly DependencyProperty CurrentInaccuracyRadiusProperty =
    DependencyProperty.Register
    (
        "CurrentInaccuracyRadius",
        typeof(double),
        typeof(PedestrianVisual),
        new UIPropertyMetadata(0.0, (s, e) =>
        {
            UpdateInaccuracyCircle((PedestrianVisual)s, (double)e.NewValue);
        })
    );

(UpdateInaccuracyCircle method should be static in this case)

If you want to stick with the instance method:

private static readonly DependencyProperty CurrentInaccuracyRadiusProperty =
    DependencyProperty.Register
    (
        "CurrentInaccuracyRadius",
        typeof(double),
        typeof(PedestrianVisual),
        new UIPropertyMetadata(0.0, (s, e) =>
        {
            ((PedestrianVisual)s).UpdateInaccuracyCircle((double)e.NewValue);
        })
    );