Cannot be used in XAML
Cannot explicitly create, reference to existing only.
The XAML syntax for properties that use a TimelineCollection is an example of an implicit collection syntax, where you omit an actual TimelineCollection object element. Instead, you generally include one or more animation types (Timeline elements) as child elements of a Storyboard. In this case the explicit syntax is not permitted; neither a <Storyboard.Children> property element nor a <TimelineCollection> object element are accepted by the parser. For more information about XAML implicit collection syntax, see XAML Syntax Overview. (Explicitly including a TimelineCollection object element is permissible XAML syntax, and might be useful if you intend to name the collection in XAML and manipulate its contents through script later.)