Crystal Reports SortDirection not working

Message

   Ok I am using Crystal report X and asp.net 2008.

I need to dynamically select a sort criteria from the asp page based on the user input.  So I created two formulas(order_by1, order_by2).  I then set these fields in the Record Sort Expert.  

In the .Net code, I am able to set the formula for order_by1 and order_by2 to a certain field that the report should be sorted on.  This works fine.  

If I try and change the SortDirection on the SortField I get an error "Cannot change the sorting field of a sort object."

If I hardcode the sort values in the Record Sort Expert then I am able to set the SortDirection in code, but can't change the field names of the sort fields.

So what am I doing wrong?

Thanks.