setting vertical scroll bar of scrollviewer to be visible

i am trying to set the verticalscrollbar visibility to be visible during a button click event. Why can't i do something like this?

before click event, XAML:

<ScrollViewer VerticalScrollBarVisibility="Hidden" Height="178.75" Width="680" Name="Scroller">

button click开发者_如何学运维 event in C#:

Scroller.VerticalScrollBarVisibility = Visibility.Visible;

-.-

It's the wrong enumeration:

Scroller.VerticalScrollBarVisibility =  ScrollBarVisibility.Visible;

You should really check the types of properties before assigning random stuff to it.