c# create or modify file/directory permission: add group read permission using asp.net with mono and apache2

Question

How can I change/set permissions of a file or directory using asp.net under mono and apache2?

I have an aspx page that needs to check if a directory exists, if not, it must be created, but with group read permissions set. Tried with DirectorySecurity, but it throws PlatformNotSupportedException.

Apache is using www-data user and all files/folders created are owned by this user, the directory permissions look like this drwx------.

The question is how they can be modified to drwxr-----?