OK so here is what I am doing:

create variable named test --- no value assigned, string datatype

right click flatfile connection, properties, expression, add expression as ConnectionString to the test variable.

So the ConnectionString should equal to variable value when the package is executed.

Run the following to run the package:

declare @ret_code int;

exec @ret_code = xp_cmdshell 'dtexec /file "test.dtsx" /decrypt xxxxxxxxxxxxx /set \Package.Variables[User::test].Properties[Value];test.txt';

select @ret_code;

Description: Cannot open the datafile "".

The error I am expecting is:

Description: Cannot open the datafile "test.txt".

Or if I supply a correct path, then the package should execute w/o errors.

\Package.Variables[User::test].Properties[Value], \Package.Variables[User::test].Value, \Package.Variables[test].Value, and \Package.Variables[test].Properties[Value] were all attempted but did not set the variable at run time.