Is the event argument that’s passed to an event handler/handler override always a non-changing snapshot taken the moment the event occurred?

No! While convention suggests a “yes” answer, no technical constrain enforces this assumption (as I discovered recently). The properties of an event argument can (and sometimes do) return information about the current (live) state of the system instead of describing the moment the event occurred.

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The program I’m working on lists transactions in a WPF Toolkit DataGrid. Each grid row displays a transaction’s date, amount, etc.  Each row in the grid needed to show the sum of all transaction amounts prior to and including that row. You might call this a per-row running total. Example:

Date       | Check #   | Payee       | Amount    | Balance
2/1/10                   Frank         10.00       10.00
3/6/10       101         John          -6.95        3.05
4/9/10       102         Tom           -1.05        2.00

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