"Should I append all the column values" - doesn't sound quite right.
Create a class representing the row data, don't convert column data to strings, if it's a date use DateTime, if its a bigint use long, and so on.
Implement the
IComparable[
^] on your row class.
Create a class implmenting IComparer<> for your class - That's easy when you have implemented IComparable.
Create a
SortedSet[
^] - you will not be allowed to add duplicates.
Or do as Christian advices - perform the operations using SQL in the database.
Best regards
Espen Harlinn