Asp.Net allows you you set the session state to readonly at individual page level or in the web.config for all pages.

But it doesn't prevent you from creating a brand new session variable on that page or modify an existing session variable. But when you try to use that session variable it is either not available or its value is not modified.

Why wouldn't the asp.net framework throw a run time error, if not compile time error, when a write attempt is made to a session on the pages where that is not allowed?