Error while trying to assign a value to a Read Write variable in SSIS package script component

Hi,

       I am trying to develop a SSIS package which will read the records from the flat file and insert them into a destination table. I have some validations written in script component. I have declared two Read Write variables with package level scope. when i try to assign a value to the variable in the script component and run the package, the package throws me an error "The collection of variables locked for read and write access is not available outside of PostExecute".

What should be done to over come the problem please help me on this regard 

Thanks

Madhavan.M