Couldn't able to use System.Web.HttpContext.Current.Request.Url.AbsoluteUri in Custom code

Hi,

I'm trying to get the absolute url of the current report(need to get HistoryID) http://<Server>/Reports/Pages/Report.aspx?ItemPath=%2fSnapShot+POC%2fTest1_SubReport&HistoryID=2011-01-10T14:32:34.

I have added System.Web.dll as reference in Report properties.

Method :

public  Function Test()  as string
 strurl  = System.Web.HttpContext.Current.Request.Url.AbsoluteUri
 return strurl  End Function

In textbox expression "=Code.Test()"

Deployed in Report Manager & it throws "#Error".

Also given FullTrust for "AspNetHostingPermission" (for System.Web.dll) in rssrvpolicy.Config,RSPreviewPolicy.Config & rsmgrpolicy.Config File

 Thanks...