Capturing HTML generated from ASP.NET

How do I best capture the HTML (in my instance, for logging) rendered by an aspx-page?

I dont want to have to write back to the page using Response.Write, since it messes up my site layout.

Using the Response.OutputStream or Response.Output’s stream results in an ArgumentException ({System.ArgumentException: Stream was not readable.)

Answers:

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Method 1

Good question, i had to try out and see if i could create a HttpModule to do what you are describing.

I didnt have any luck trying to read from the responsestream, but using the ResponseFilter gave me a way to capture the content.

The following code seems to work pretty good, and i figured maybe you could use the code as a base. But remember this is just something i threw together fast, it has not been tested in any way. So dont use it in any production environment without proper reviewing/testing and such. Feel free to comment on it though 😉

public class ResponseLoggerModule : IHttpModule
{
    private class ResponseCaptureStream : Stream
    {
        private readonly Stream _streamToCapture;
        private readonly Encoding _responseEncoding;

        private string _streamContent;
        public string StreamContent
        {
            get { return _streamContent; }
            private set
            {
                _streamContent = value;
            }
        }

        public ResponseCaptureStream(Stream streamToCapture, Encoding responseEncoding)
        {
            _responseEncoding = responseEncoding;
            _streamToCapture = streamToCapture;

        }

        public override bool CanRead
        {
            get { return _streamToCapture.CanRead; }
        }

        public override bool CanSeek
        {
            get { return _streamToCapture.CanSeek; }
        }

        public override bool CanWrite
        {
            get { return _streamToCapture.CanWrite; }
        }

        public override void Flush()
        {
            _streamToCapture.Flush();
        }

        public override long Length
        {
            get { return _streamToCapture.Length; }
        }

        public override long Position
        {
            get
            {
                return _streamToCapture.Position;
            }
            set
            {
                _streamToCapture.Position = value;
            }
        }

        public override int Read(byte[] buffer, int offset, int count)
        {
            return _streamToCapture.Read(buffer, offset, count);
        }

        public override long Seek(long offset, SeekOrigin origin)
        {
            return _streamToCapture.Seek(offset, origin);
        }

        public override void SetLength(long value)
        {
            _streamToCapture.SetLength(value);
        }

        public override void Write(byte[] buffer, int offset, int count)
        {
            _streamContent += _responseEncoding.GetString(buffer);
            _streamToCapture.Write(buffer, offset, count);
        }

        public override void Close()
        {
            _streamToCapture.Close();
            base.Close();
        }
    }

    #region IHttpModule Members

    private HttpApplication _context;
    public void Dispose()
    {

    }

    public void Init(HttpApplication context)
    {
        _context = context;

        context.PreRequestHandlerExecute += new EventHandler(context_PreRequestHandlerExecute);
        context.PreSendRequestContent += new EventHandler(context_PreSendRequestContent);
    }

    void context_PreRequestHandlerExecute(object sender, EventArgs e)
    {
        _context.Response.Filter = new ResponseCaptureStream(_context.Response.Filter, _context.Response.ContentEncoding);
    }

    void context_PreSendRequestContent(object sender, EventArgs e)
    {
        ResponseCaptureStream filter = _context.Response.Filter as ResponseCaptureStream;

        if (filter != null)
        {
            string responseText = filter.StreamContent;

            // Logging logic here
        }
    }

    #endregion
}

Method 2

Many load testers will allow you to log the HTTP responses generated, but bear in mind with ASP.NET those could be some very large log-files.

Edit: Response.Filter as per Tom Jelen’s code is designed to give this kind of oversight and Response.Outputstream is otherwise unreadable.

Edit 2: For a page rather than a HTTPModule

public class ObserverStream : Stream
{
  private byte[] buffer = null;
  private Stream observed = null;

  public ObserverStream (Stream s)
  {
    this.observed = s;
  }

  /* important method to extend #1 : capturing the data */
  public override void Write(byte[] buffer, int offset, int count)
  {
    this.observed.Write(buffer, offset, count);
    this.buffer = buffer; //captured!
  }

  /* important method to extend #2 : doing something with the data */
  public override void Close()
  {
    //this.buffer available for logging here!
    this.observed.Close();
  }

  /* override all the other Stream methods/props with this.observed.method() */

  //...

}

and in your Page_Load (or before your response is written anyway)

Response.Filter = new ObserverStream(Response.Filter);

Method 3

One way to to make server-side XMLHTTP request to your own server. Grab the result and save it to a file or DB.

Alternately you can use AJAX on the client, grab the result, and POST it back to the server.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0