How to loop through a checkboxlist and to find what's checked and not checked?

This will give a list of selected

List<ListItem> items =  checkboxlist.Items.Cast<ListItem>().Where(n => n.Selected).ToList();

This will give a list of the selected boxes' values (change Value for Text if that is wanted):

var values =  checkboxlist.Items.Cast<ListItem>().Where(n => n.Selected).Select(n => n.Value ).ToList()

for (int i = 0; i < clbIncludes.Items.Count; i++)
  if (clbIncludes.GetItemChecked(i))
    // Do selected stuff
  else
    // Do unselected stuff

If the the check is in indeterminate state, this will still return true. You may want to replace

if (clbIncludes.GetItemChecked(i))

with

if (clbIncludes.GetItemCheckState(i) == CheckState.Checked)

if you want to only include actually checked items.

Try something like this:

foreach (ListItem listItem in clbIncludes.Items)
{
    if (listItem.Selected) { 
        //do some work 
    }
    else { 
        //do something else 
    }
}