How do I get the current Url from within a FilterAttribute?

I am writing an Authorize filter attribute adn I'm having trouble figuring out how to get the current url as a string so I can pass it as a parameter to the LogOn action. The goal is that if a user successfully logs on, they will be redirected to the page they were originally trying to access.

public override void OnAuthorization(AuthorizeContext filterContext)
{
    base.OnAuthorization(filterContext);

    ... my auth code ...
    bool isAuth ;
    ... my auth code ...

    if(!isAuth)
    {
        filterContext.Result = new RedirectToRouteResult(
            new RouteValueDictionary { 
                { "Area", "" }, 
                { "Controller", "Account" }, 
                { "Action", "LogOn" },
                { "RedirectUrl", "/Url/String/For/Currnt/Request" } // how do I get this?
            }
        );
    }
}

How do I get the full string Url from the current request?

Try:

var url = filterContext.HttpContext.Request.Url;

To get the complete URL you can try as suggested by the @rboarman but usually the RedirectUrl will be the relative url and for that you have to try the the RawUrl property of the Request object.

filterContext.HttpContext.Request.Url  ===> http://somesite.com/admin/manage

filterContext.HttpContext.Request.RawUrl ====> /admin/manage

EDITED: Fixed the second example

In my specific case I was after the UrlReferrer URL.

filterContext.HttpContext.Request.UrlReferrer

This one let me redirect the user back to the page he was before trying to access an action he doesn't have permission to access.