Validating the referrer url against a collection of route value dictionaries

gistfile1.cs

public static string PreviousUrl(this UrlHelper url, string defaultUrl, RouteValueDictionary[] validRoutes)
        {
            var previousUrl = url.GetPreviousUrl(false);

            if (previousUrl == null)
                return defaultUrl;

            var request = url.RequestContext.HttpContext.Request;
            var stubContext = new StubHttpContextForRouting(request.ApplicationPath,
                VirtualPathUtility.ToAppRelative(previousUrl.AbsolutePath));

            var routeData = RouteTable.Routes.GetRouteData(stubContext);

            if (validRoutes.Any(rv => Matches(routeData.Values, rv, "controller", "action")))
                return previousUrl.AbsoluteUri;

            return defaultUrl;
        }

        internal static bool Matches(RouteValueDictionary d1, RouteValueDictionary d2, params string[] keys)
        {
            bool match = true;   
            
            foreach (var key in keys)
            {
                object val1, val2;
                if (d1.TryGetValue(key, out val1) && d2.TryGetValue(key, out val2))
                {
                    if (!(val1 as string).Equals((val2 as string), StringComparison.InvariantCultureIgnoreCase))
                    {
                        match = false;
                        break;
                    }
                }
            }

            return match;
        }

        internal static Uri GetPreviousUrl(this UrlHelper url, bool allowExternal)
        {
            var httpRequest = url.RequestContext.HttpContext.Request;
            var previousUrl = httpRequest.UrlReferrer;

            if (previousUrl != null && (allowExternal || url.IsLocalUrl(previousUrl.AbsoluteUri)))
            {
                return previousUrl;
            }

            return null;
        }