Capturing HTML output with a controller action filter

Question

I've got the following filter in place on an action to capture the HTML output, convert it to a string, do some operations to modify the string, and return a ContentResult with the new string. Unfortunately, I keep ending up with an empty string.

private class UpdateFilter : ActionFilterAttribute
    {
        private Stream stream;

        public override void OnActionExecuting(ActionExecutingContext filterContext)
        {
            stream = filterContext.HttpContext.Response.Filter;
            stream = new MemoryStream();
            filterContext.HttpContext.Response.Filter = stream;
        }

        public override void OnResultExecuted(ResultExecutedContext filterContext)
        {
            StreamReader responsereader = new StreamReader(filterContext.HttpContext.Response.Filter);  //empty stream? why?
            responsereader.BaseStream.Position = 0;
            string response = responsereader.ReadToEnd();
            ContentResult contres = new ContentResult();
            contres.Content = response;
            filterContext.Result = contres;
        }
    }

I've pinned down that StreamReader(stream).ReadToEnd() returns an empty string, but I can't figure out why.

Any ideas how to fix this?

EDIT: I've changed the OnActionExecuted to OnResultExecuted, and now it is called after the View has been generated, but the stream is still empty!

Answer 1

I solved this by hijacking the HttpWriter, and having it write into a StringBuilder rather than the response, and then doing whatever needs to be done to/with the response before writing it to the output.

 private class UpdateFilter : ActionFilterAttribute
    {
        private HtmlTextWriter tw;
        private StringWriter sw;
        private StringBuilder sb;
        private HttpWriter output;

        public override void OnActionExecuting(ActionExecutingContext filterContext)
        {
            sb = new StringBuilder();
            sw = new StringWriter(sb);
            tw = new HtmlTextWriter(sw);
            output = (HttpWriter)filterContext.RequestContext.HttpContext.Response.Output;
            filterContext.RequestContext.HttpContext.Response.Output = tw;
        }

        public override void OnResultExecuted(ResultExecutedContext filterContext)
        {
            string response = sb.ToString();
            //response processing
            output.Write(response);
        }
    }