ASP.NET C# CheckBoxList Control Issues.

I'm trying to implement a filter for a gridview.
I have created a cbl control with 5 items.
I have 1 set as an ALL box and when that gets checked the others are automatically de-selected.
Now what I need to do is set it up so that if any other items are checked they clear the "ALL" box. The Problem I am running into is the cbl.Items[i].Selected property tells me if a box is selected, NOT if it was clicked. I know there isn't an "OnClick" event to check but is there a way I can pass the index# of the box to the handler constructor?