How to re-render Window layout at run-time after collapsing a child UI element

Hi,

I want to collapse or hide a child element that is in a StackPanel on Main WPF window at run-time on a particular condition.  When I collapse after the Window is initialized, the element is not visible but I still see the empty space.  Is there a way to re-render the Window layout after I hide or collapse that child element ?  I am also fine to conditionally create that element in XAML but not sure how to write C# code in XAML to determine if that condition is true or not ?

Please help.

Thanks.