well i am sorry
i asked to quickly :-(
from delphi help:
String is one of those data types which need finalization, and as such they cannot be stored in a File type.
One simple solution, for the case of String, is to redeclare the type as an array of characters. For other cases which require finalization, it becomes increasingly difficult to maintain a binary file structure with standard Pascal features, such as 'file of'. In these situations, it is probably easier to write specialized file I/O routines.