Why Doesn't My Dependency Property's Setter Fire When Set in XAML?

Designer-Triggered Property Change Detection

When setting the value of the IsClosed property during runtime, the OnIsClosedChanged() method is invoked as expected. However, when the property is set in XAML by the designer, the method remains uncalled. This is because dependency properties set in XAML or other non-runtime contexts bypass setter methods.

To address this issue, you can register a PropertyChangedCallback with property metadata. This callback allows you to specify a custom behavior to execute when the value of the property changes.

public static readonly DependencyProperty IsClosedProperty =
    DependencyProperty.Register(
        "IsClosed", typeof(bool), typeof(GroupBox),
        new FrameworkPropertyMetadata(false,
            FrameworkPropertyMetadataOptions.AffectsRender,
            (o, e) => ((GroupBox)o).OnIsClosedChanged()));

This code creates a new dependency property named IsClosedProperty and registers a property metadata callback that executes the OnIsClosedChanged() method when the property value changes. With this change, both runtime and XAML property updates will trigger the desired behavior, ensuring that your UI responds appropriately.

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