Q:

How can I get my CheckBoxList to have three possible states?

How can I get my CheckBoxList to have three possible states ? More precisely, is there a way to have three possible states for each checkbox : checked, unchecked, undefined (in most GUIs this is represented as a full square).
Alternatively, do you recommend another control that would meet these needs ?
UPDATE : Ok, given that HTML does not support tri-state checkboxes, I'm looking for a way to 'CSS' (color fill, highlight, etc...) the checkboxes of the items that are in that 'undefined' state from my server point of view. 

A:

Not all GUIs represent undefined as a full square. That's why it's going to be clearer to use one of the following:
1. Radio boxes
2. Dropdown
3. ListBox 

A:

I have decided to use CheckBoxList anyway, since I need two states accessible by the user, and a third one not accessible by the user but to display inconsistencies. 
The way I have solved this is through Javascript + CSS. Some bits of ideas here: http://css-tricks.com/indeterminate-checkboxes/

