ASP.MVC Filter that pass along Google Analytics utm querystring parameters

Question

I'm trying to build a filter that when applied pass alongs "utm_" querystring params

This is what I came up with:

public class PassAlongParametersFilter : FilterAttribute, IResultFilter
{
    public string Filter { get; set; }

    public PassAlongParametersFilter()
    {
        Filter = "*";
    }

    public void OnResultExecuting(ResultExecutingContext filterContext)
    {
    }

    public void OnResultExecuted(ResultExecutedContext filterContext)
    {
        if (filterContext.Result is RedirectToRouteResult)
        {
            var action = (RedirectToRouteResult) filterContext.Result;

            var qs = filterContext.RequestContext.HttpContext.Request.QueryString;

            var regex = StringUtils.WildcardToRegex(Filter);

            var routeValues = action.RouteValues;

            qs.AllKeys.Where(e => Regex.IsMatch(e, regex)).ForEach(s => routeValues[s] = qs[s]);

            filterContext.Result = new RedirectToRouteResult(action.RouteName, routeValues, action.Permanent);
        }
    }
}

I can see the routeValues being populated correctly while debugging but the utm params aren't included in the resulted url.

This is the action with filter:

[HttpPost]
[AllowAnonymous]
[PassAlongParametersFilter(Filter = "utm_*")]
[Route("konto/registrera/externt/{campaignCode}")]
public ActionResult SimpleRegisterExternal(string email, string campaignCode)
{

And this is the action that I try to redirect to:

[AllowAnonymous]
[Route("konto/registrera/tack")]
public ActionResult RegisterThanks(RegisterThanksViewModel model)
{

I'm guessing there's something with the routing?

Answer 1

Ok so I figured it out. Replacing the Result in OnResultExecuted is too late. Here's what I ended up with:

public class PassAlongParametersFilter : ActionFilterAttribute
    {
        public string Filter { get; set; }

        public PassAlongParametersFilter()
        {
            Filter = "*";
        }

        public override void OnActionExecuted(ActionExecutedContext filterContext)
        {
            if (filterContext.Result is RedirectToRouteResult)
            {
                var action = (RedirectToRouteResult)filterContext.Result;

                var qs = filterContext.RequestContext.HttpContext.Request.QueryString;

                var regex = StringUtils.WildcardToRegex(Filter);

                var routeValues = action.RouteValues;

                qs.AllKeys.Where(e => Regex.IsMatch(e, regex)).ForEach(s => routeValues[s] = qs[s]);

                filterContext.Result = new RedirectToRouteResult(action.RouteName, routeValues, action.Permanent);
            }
        }

    }