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

Answer 1

I think I've developed a pretty good way to do this.

• Replace the Reponse Filter with a custom one
• This filter takes a delegate to an abstract method which takes a stream
• This the delegate, and hence the abstract method are called on the close of the stream, i.e. when all the HTML is available
• Override the OnClose method and play with the stream as you like.

---

public abstract class ReadOnlyActionFilterAttribute : ActionFilterAttribute
{
    private delegate void ReadOnlyOnClose(Stream stream);

    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        filterContext.HttpContext.Response.Filter = new OnCloseFilter(
            filterContext.HttpContext.Response.Filter, 
            this.OnClose);
        base.OnActionExecuting(filterContext);
    }

    protected abstract void OnClose(Stream stream);

    private class OnCloseFilter : MemoryStream
    {
        private readonly Stream stream;

        private readonly ReadOnlyOnClose onClose;

        public OnCloseFilter(Stream stream, ReadOnlyOnClose onClose)
        {
            this.stream = stream;
            this.onClose = onClose;
        }

        public override void Close()
        {
            this.Position = 0;
            this.onClose(this);
            this.Position = 0;
            this.CopyTo(this.stream);
            base.Close();
        }
    }
}

You can then derive from this to another attribute to access the stream and get the HTML:

public class MyAttribute : ReadOnlyActionFilterAttribute
{
    protected override void OnClose(Stream stream)
    {
        var html = new HtmlDocument();
        html.Load(stream);
        // play with html
    }
}