Ho to grant permissions for the programm code

Greetings to all!

I'm searching for a technique to set the file system permissions of aspx pages.

Usually we set the permissions for the loged in users, but it's not the case.

I need to implement following:

execution of the code

My.Computer.FileSystem.WriteAllText("C:\temp\hello.txt", "write access", False) 
from the page 'main.aspx' should be successful, but execution of the same code  from the page 'restricted.aspx' should fail.

I have prepared a sample with two aspx pages.If you need I can post this files.

I am looking forward to your suggestions. ...