You need to store data in a file, but your application doesn’t run with the required FileIOPermission.
Use a user-specific isolated store.
The .NET Framework includes support for isolated storage, which allows you to read and write to a user-specific virtual file system that the common language runtime manages. When you create isolated storage files, the data is automatically serialized to a unique location in the user profile path (typically a path like c:\document and settings\[username]\local settings\application data\isolated storage\[guid_identifier]).
One reason you might use isolated storage is to give an untrusted application limited ability to store data. For example, the default common language ...