[alicedr]

Добавлю сюда еще неработающий метод:

[FormDataSourceEventHandler(formDataSourceStr(SalesTable, InventTable), FormDataSourceEventType::Writing)]
    public static void InventTable_OnWriting(FormDataSource sender, FormDataSourceEventArgs e)
    {
        // doesn't work
        FormDataSourceCancelEventArgs ce = e as FormDataSourceCancelEventArgs; //converting error here
        Ce.cancel(); //Ce.cancel(1); ?
       // it does work for form control:
       // FormControlCancelableSuperEventArgs ce = e as FormControlCancelableSuperEventArgs;
       // ce.CancelSuperCall();
    }

И пару ссылок про полезные извращения:
https://ievgensaxblog.wordpress.com/...ut-overlaying/
https://docs.microsoft.com/en-us/dyn...m-data-sources