Look at the SelectedItems property, and iterate through that to see which items are selected.
If you want to go through all items you can compare the two collections (MyListBox.Items and MyListBox.SelectedItems) and see which ones match.
something like:
foreach(Item item in MyListBox.Items)
if(MyListBox.SelectedItems.Contains(item)
MyObject.Value = true;
else
MyObject.Value = false;
Overkill though really! I guess there is a purpose if you want to do something to all items which are not selected though, is that what you are looking to do?
There are much better ways to do this though - Randolpho is correct, databinding would be a better way to go about this depending on how your data is organised/input and how big the listbox is.