Httpcontext.current.request is null in authorisation.cs for web portal url

[GD22567]

My custom code is working fine when accessing web service url (Note: I have used a value from httpcontext.current.request in authorisation.cs) .But when I am running same code for web portal url it is throwing error .
An error occured when invoking authorisation extension.

Error is coming because httpcontext.current.request is null when reaching authorisation.cs for web portal url.

I am trying to figure out in case of web portal why only username is being transmitted to authorisation.cs and not httpcontext values.How it is different for web service url.
If I remove httpcontext from authorisation.cs ,it works fine for webportal as well.
Trying to check if along with username if I can pass httpcontext value as well from authentication.cs to authorisation.cs.
Any suggestion is appreciated.