Rss20FeedFormatter Ignores TextSyndicationContent

问题:

While using the Rss20FeedFormatter class in a WCF project, I was trying to wrap the content of my description elements with a <![CDATA[ ]]> section. I found that no matter what I did, the HTML content of the description elements was always encoded and the CDATA section was never added. After peering into the source code of Rss20FeedFormatter, I found that when building the Summary node, it basically creates a new TextSyndicationContent instance which wipes out whatever settings were previously specified (I think).

My Code

public class CDataSyndicationContent : TextSyndicationContent
{
    public CDataSyndicationContent(TextSyndicationContent content)
        : base(content)
    {
    }

    protected override void WriteContentsTo(System.Xml.XmlWriter writer)
    {
        writer.WriteCData(Text);
    }
}

... (The following code should wrap the Summary with a CDATA section)

SyndicationItem item = new SyndicationItem();
item.Title = new TextSyndicationContent(name);
item.Summary = new CDataSyndicationContent(
                   new TextSyndicationContent(
                         "<div>This is a test</div>", 
                         TextSyndicationContentKind.Html));

Rss20FeedFormatter Code (AFAIK, the above code does not work because of this logic)

...
else if (reader.IsStartElement("description", ""))
   result.Summary = new TextSyndicationContent(reader.ReadElementString());
...

As a workaround, I've resorted to using the RSS20FeedFormatter to build the RSS, and then patch the RSS manually. For example:

        StringBuilder buffer = new StringBuilder();
        XmlTextWriter writer = new XmlTextWriter(new StringWriter(buffer));
        feedFormatter.WriteTo(writer ); // feedFormatter = RSS20FeedFormatter

        PostProcessOutputBuffer(buffer);
        WebOperationContext.Current.OutgoingResponse.ContentType = 
                                   "application/xml; charset=utf-8";
        return new MemoryStream(Encoding.UTF8.GetBytes(buffer.ToString()));      

...

    public void PostProcessOutputBuffer(StringBuilder buffer)
    {
        var xmlDoc = XDocument.Parse(buffer.ToString());
        foreach (var element in xmlDoc.Descendants("channel").First()
                                      .Descendants("item")
                                      .Descendants("description"))
        {
            VerifyCdataHtmlEncoding(buffer, element);
        }

        foreach (var element in xmlDoc.Descendants("channel").First()
                                      .Descendants("description"))
        {
            VerifyCdataHtmlEncoding(buffer, element);
        }

        buffer.Replace(" xmlns:a10=\"http://www.w3.org/2005/Atom\"",
                       " xmlns:atom=\"http://www.w3.org/2005/Atom\"");
        buffer.Replace("a10:", "atom:");
    }

    private static void VerifyCdataHtmlEncoding(StringBuilder buffer,
                                                XElement element)
    {
        if (!element.Value.Contains("<") || !element.Value.Contains(">"))
        {
            return;
        }

        var cdataValue = string.Format("<{0}><![CDATA[{1}]]></{2}>",
                                       element.Name,
                                       element.Value, 
                                       element.Name);
        buffer.Replace(element.ToString(), cdataValue);
    }

The idea for this workaround came from the following location, I just adapted it to work with WCF instead of MVC. http://localhost:8732/Design_Time_Addresses/SyndicationServiceLibrary1/Feed1/

I'm just wondering if this is simply a bug in Rss20FeedFormatter or is it by design? Also, if anyone has a better solution, I'd love to hear it!

回答1:

Well @Page Brooks, I see this more as a solution then as a question :). Thanks!!! And to answer your question ( ;) ), yes, I definitely think this is a bug in the Rss20FeedFormatter (though I did not chase it as far), because had encountered precisely the same issue that you described.

You have a 'localhost:8732' referral in your post, but it wasn't available on my localhost ;). I think you meant to credit the 'PostProcessOutputBuffer' workaround to this post: http://damieng.com/blog/2010/04/26/creating-rss-feeds-in-asp-net-mvc

Or actually it is not in this post, but in a comment to it by David Whitney, which he later put in a seperate gist here: https://gist.github.com/davidwhitney/1027181

Thank you for providing the adaption of this workaround more to my needs, because I had found the workaround too, but was still struggling to do the adaptation from MVC. Now I only needed to tweak your solution to put the RSS feed to the current Http request in the .ashx handler that I was using it in.

Basically I'm guessing that the fix you mentioned using the CDataSyndicationContent, is from feb 2011, assuming you got it from this post (at least I did): SyndicationFeed: Content as CDATA?

This fix stopped working in some newer ASP.NET version or something, due to the code of the Rss20FeedFormatter changing to what you put in your post. This code change might as well have been an improvement for other stuff that IS in the MVC framework, but for those using the CDataSyndicationContent fix it definitely causes a bug!

回答2:

I found the code for Cdata elsewhere

    public class CDataSyndicationContent : TextSyndicationContent
{
    public CDataSyndicationContent(TextSyndicationContent content)
        : base(content)
    {
    }

    protected override void WriteContentsTo(System.Xml.XmlWriter writer)
    {
        writer.WriteCData(Text);
    }
}

Code to call it something along the lines:

item.Content = new Helpers.CDataSyndicationContent(new TextSyndicationContent("<span>TEST2</span>", TextSyndicationContentKind.Html));

However the "WriteContentsTo" function wasn't being called.

Instead of Rss20FeedFormatter I tried Atom10FeedFormatter - and it worked! Obviously this gives Atom feed rather than traditional RSS - but worth mentioning.

Output code is:

//var formatter = new Rss20FeedFormatter(feed);
    Atom10FeedFormatter formatter = new Atom10FeedFormatter(feed);
    using (var writer = XmlWriter.Create(response.Output, new XmlWriterSettings { Indent = true }))
    {
        formatter.WriteTo(writer);
    }

回答3:

string stylesheet = @"<xsl:stylesheet version=""1.0"" xmlns:xsl=""http://www.w3.org/1999/XSL/Transform""><xsl:output cdata-section-elements=""description"" method=""xml"" indent=""yes""/></xsl:stylesheet>";
XmlReader reader = XmlReader.Create(new StringReader(stylesheet));
XslCompiledTransform t = new XslCompiledTransform(true);
t.Load(reader);

using (MemoryStream ms = new MemoryStream())
{
    XmlWriter writer = XmlWriter.Create(ms, t.OutputSettings);
    rssFeed.WriteTo(writer);  // rssFeed is Rss20FeedFormatter 
    writer.Flush();
    ms.Position = 0;
    string niko = Encoding.UTF8.GetString(ms.ToArray());
}

I'm sure someone pointed this out already but this a stupid workaround I used. t.OutputSettings is of type XmlWriterSettings with cdataSections being populated with a single XmlQualifiedName "description".

Hope it helps someone else.