You are using incorrect syntax for DependencyProperty CLR wrappers(getter/setter).
Use the following correct code:
public static readonly DependencyProperty VideoPlayerSourceProperty = DependencyProperty.Register("VideoPlayerSource",
typeof(System.Uri), typeof(Player),
new PropertyMetadata(null, (dObj, e) => ((Player)dObj).OnVideoPlayerSourceChanged((System.Uri)e.NewValue)));
public System.Uri VideoPlayerSource {
get { return (System.Uri)GetValue(VideoPlayerSourceProperty); } // !!!
set { SetValue(VideoPlayerSourceProperty, value); } // !!!
}
void OnVideoPlayerSourceChanged(System.Uri source) {
mediaElement.Source = source;
}