public static RouteValueDictionary RoutesToCurrentPage( this HtmlHelper helper )
{
    var routeValues = new RouteValueDictionary( helper.ViewContext.RouteData.Values );
    var queryString = helper.ViewContext.HttpContext.Request.QueryString;
    foreach ( string  key in queryString.Keys )
    {
        routeValues[key] = queryString[key];
    }
    return routeValues;
}

//Usage:

//In controller
var routeValues = Html.RoutesToCurrentPage();
routeValues["someParam"] = "newValue";

// In View
Html.ActionLink("Action", "Controller", routeValues);