Dependency property changed example

public double ScaleFactor
{
    get { return (double)GetValue(ScaleFactorProperty); }
    set { SetValue(ScaleFactorProperty, value); }
}

// Using a DependencyProperty as the backing store for ScaleFactor.  This enables animation, styling, binding, etc...
public static readonly DependencyProperty ScaleFactorProperty =
    DependencyProperty.Register("ScaleFactor", typeof(double), typeof(ToolBlockControl), 
        new UIPropertyMetadata(1.0,new PropertyChangedCallback(ScaleFactorChangedCallBack)));


private static void ScaleFactorChangedCallBack(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
    var toolBlockControl = obj as ToolBlockControl;
    double scaleFactor = (double)args.NewValue;
    toolBlockControl.Width = toolBlockControl.mActualWidth * scaleFactor;
    toolBlockControl.Height = toolBlockControl.mActualHeight * scaleFactor;
}