Dynamically Update ListBox Item

I have as a ListBox item a reference to a ScrollingLine class, in which I
override ToString() to return the part of text currently visible in a
scrolling window.  How do I get my ListBox to update its display of that
string?  Does it make a copy of the ToString() result of each item when
initialising, or does it just not know to update and maybe I could
invalidate it?