checked or unchecked

My datagridview has checkbox columns, one for "Odd" and one for "Even".  When user checks Odd, I want to uncheck Even, because they can't both be checked.  I have 2 questions:

1. What event should I start with?
2. How do I determine whether user has checked on or off?

So far I've tried using the CellValueChanged and MouseClick events.  I'm trying to determine whether the user has checked or unchecked the box like this:

Dim ck As Boolean = CBool(datagridview.Rows((e.RowIndex)).Cells(e.ColumnIndex).Value)

With this, ck is always false.  Why?  Should I be catching this in another event?Start Free Trial