WPF – Change Property Value in Setter of Another Property

Question

In the following example of my custom control, why I can't change a TransparentColor property in a setter of a SelectedColor property? A solution with callback method works fine, what's the difference between them in the case of change another property?

public class MyColorPicker : Control
{
    static MyColorPicker()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(MyColorPicker), new FrameworkPropertyMetadata(typeof(MyColorPicker)));
    }

    public static readonly DependencyProperty SelectedColorProperty =
            DependencyProperty.Register("SelectedColor", typeof(Color), typeof(MyColorPicker), new PropertyMetadata(Color.FromRgb(0, 201, 201), OnSelectedColorChange));

    private static void OnSelectedColorChange(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        var colorPicker = (MyColorPicker)sender;
        var selectedColor = colorPicker.SelectedColor;
        colorPicker.TransparentColor = Color.FromArgb(0, selectedColor.R, selectedColor.G, selectedColor.B);
    }

    public Color SelectedColor
    {
        get { return (Color)GetValue(SelectedColorProperty); }
        set
        {
            // Why this not working? BorderStopColor = Color.FromArgb(0, value.R, value.G, value.B);
            SetValue(SelectedColorProperty, value);
        }
    }

    private static readonly DependencyPropertyKey TransparentColorPropertyKey =
            DependencyProperty.RegisterReadOnly("TransparentColor", typeof(Color), typeof(MyColorPicker), new PropertyMetadata(Color.FromArgb(0, 0, 201, 201)));

    public static readonly DependencyProperty TransparentColorProperty = TransparentColorPropertyKey.DependencyProperty;

    public Color TransparentColor
    {
        get { return (Color)GetValue(TransparentColorProperty); }
        protected set { SetValue(TransparentColorPropertyKey, value); }
    }
}

score 1 · Accepted Answer · answered Jul 04 '17 at 09:41

The setter of a CLR wrapper for a dependency property should only call the SetValue method to set the actual value of the dependency property.

Any other logic should be implemented in the callback.

Also note that the CLR property wrappers are bypassed at runtime when setting dependency properties in XAML (but the callbacks are not):

Why are .NET property wrappers bypassed at runtime when setting dependency properties in XAML?

WPF – Change Property Value in Setter of Another Property

1 Answers1