Using datatable Group by 2 columns and sort by 1 column

I have data of 3k records. i want to groupby say column A and B and sort by column C. and perform some tasks on each group.
Please guide me on the same

find starter help here:
GroupBy_2Cols_OrderGroupBy-1Col.xaml (10.1 KB)

My column 3 is a date time so i have given convert.todatetime()

but i am getting an error
Object cannot be cast from DBNull to other types

please show your data samples so we can help more specific
empty strings will fail within datetime parsing and it looks that some rows do miss valid Col3 values