Convert Ifieldobject type from string to number in crystal report

Question

Hi , friends ,
I make crystal report dynamically but with this , all fileldobject types are string while some of them should be numeric .

For example , i want to create a report with four fields : Product_no,Product_Name, Unit, Quantity
    Firstly , i create a datatable dt with 4 columns : column1,column2,column3,column4
   Then ,  data for crystal report is from this datatable .
So , what should i do to change type of Ifieldobject column4 ( quantity) to numeric instead of string
Thanks so much ,