Why don't you use a binding ? That's precisely what they're designed for...
If, for some reason, you can't use a binding, you can add a handler to be notified when the value of the property changes :
var topDescriptor = DependencyPropertyDescriptor.FromProperty(Canvas.TopProperty, typeof(Rectangle));
var leftDescriptor = DependencyPropertyDescriptor.FromProperty(Canvas.LeftProperty, typeof(Rectangle));
topDescriptor.AddValueChanged(rectangle, rectangle_PositionChanged);
leftDescriptor.AddValueChanged(rectangle, rectangle_PositionChanged);
...
private void rectangle_PositionChanged(object sender, EventArgs e)
{
...
}
