Forget about writing Atom or RSS XML handling code ever again

A *very* welcome addition to .NET 3.5, which just went RTM for MSDN subscribers and trial for the rest before general availability early next year: System.ServiceModel.Syndication.

This namespace, which lives in the System.ServiceModel.Web.dll assembly which provides the WCF Syndication functionality, contains useful classes for working with feeds and items. I won't go over the Architecture of Syndication, How the WCF Syndication Object Model Maps to Atom and RSS, How to: Create a Basic RSS Feed, How to: Create a Basic RSS Feed, How to: Expose a Feed as both Atom and RSS or the basics of Syndication Extensibility. All those links provide enough to get you started.

The typical usage is:

• Create XmlReader/XmlWriter of the feed you will be reading/writing.
• Create Atom10FeedFormatter/Rss20FeedFormatter which knows how to read/write the given format (there are Atom10ItemFormatter and Rss20ItemFormatter too)
• Read or write a SyndicationFeed or a SyndicationFeedItem.

The only thing I felt was missing was a factory that abstracts me from having to decide which formatter to create for a given source: the factory should be able to determine this automatically depending on the first element of the feed/item.

With the factory in place, reading a feed without caring for its format, is as simple as:

using (XmlReader reader = XmlReader.Create(atomFeedUrl))
{
    SyndicationFeedFormatter formatter = SyndicationFormatterFactory.CreateFeedFormatter(reader);
    formatter.ReadFrom(reader);
    SyndicationFeed feed = formatter.Feed;
}

Note that there's nothing in my code that knows about RSS or Atom.

Processing of the item and extensions is very friendly and quite flexible, making for a very nice platform to base syndication extension processing plugins:

    foreach (SyndicationItem item in feed.Items)
    {
        // ... process item

        foreach (SyndicationElementExtension extension in item.ElementExtensions)
        {
            // ... process extensions
            // example: SSE sync
            if (extension.OuterNamespace == Sync.NamespaceUri)
            {
                // NOTE: we don't need to pass an explicit XmlSerializer because we implement 
                // IXmlSerializable, so the library figures out it needs one :)
                Sync sync = extension.GetObject<Sync>();
                // ... do something with the extension
            }
            else if (extension.OuterName == "Mock")
            {
                // this extension does not implement IXmlSerializable neither is a WCF data 
                // contract, so we need to pass an XmlSerializer
                Mock m = extension.GetObject<Mock>(new XmlSerializer(typeof(Mock)));
                // ... do something with the extension.
            }
        }
    }

So here goes such a factory:

/// <summary>
/// Creates formatters for RSS 2.0 and Atom 1.0 according to the input content.
/// </summary>
/// <remarks>
/// <see cref="http://www.clariusconsulting.net/kzu">Created by Daniel Cazzulino.</see>
/// </remarks>
public static class SyndicationFormatterFactory
{
    static XmlReaderSettings settings;

    static SyndicationFormatterFactory()
    {
        // Makes the processing faster for the readers we create.
        settings = new XmlReaderSettings();
        settings.IgnoreComments = true;
        settings.IgnoreProcessingInstructions = true;
        settings.IgnoreWhitespace = true;
        settings.CheckCharacters = true;
        settings.CloseInput = true;
    }

    /// <summary>
    /// Creates a <see cref="SyndicationFeedFormatter"/> according to the 
    /// input format.
    /// </summary>
    /// <param name="uriString">Feed location</param>
    /// <exception cref="NotSupportedException">The input does not contain a valid RSS 2.0 or Atom 1.0 feed.</exception>
    public static SyndicationFeedFormatter CreateFeedFormatter(string uriString)
    {
        using (XmlReader reader = XmlReader.Create(uriString, settings))
            return CreateFeedFormatter(reader);
    }

    /// <summary>
    /// Creates a <see cref="SyndicationFeedFormatter"/> according to the 
    /// input format.
    /// </summary>
    /// <param name="uri">Feed location</param>
    /// <exception cref="NotSupportedException">The input does not contain a valid RSS 2.0 or Atom 1.0 feed.</exception>
    public static SyndicationFeedFormatter CreateFeedFormatter(Uri uri)
    {
        using (XmlReader reader = XmlReader.Create(uri.ToString(), settings))
            return CreateFeedFormatter(reader);
    }

    /// <summary>
    /// Creates a <see cref="SyndicationFeedFormatter"/> according to the 
    /// input format.
    /// </summary>
    /// <param name="reader">Feed source</param>
    /// <exception cref="NotSupportedException">The input does not contain a valid RSS 2.0 or Atom 1.0 feed.</exception>
    public static SyndicationFeedFormatter CreateFeedFormatter(XmlReader reader)
    {
        if (reader.ReadState == ReadState.Initial)
        {
            reader.MoveToContent();
        }

        Rss20FeedFormatter rss = new Rss20FeedFormatter();
        if (rss.CanRead(reader))
        {
            return rss;
        }

        Atom10FeedFormatter atom = new Atom10FeedFormatter();
        if (atom.CanRead(reader))
        {
            return atom;
        }

        throw new NotSupportedException("Invalid feed root element: " + reader.Name);
    }

    /// <summary>
    /// Creates a <see cref="SyndicationItemFormatter"/> according to the 
    /// input format.
    /// </summary>
    /// <param name="uriString">Item location</param>
    /// <exception cref="NotSupportedException">The input does not contain a valid RSS 2.0 or Atom 1.0 item.</exception>
    public static SyndicationItemFormatter CreateItemFormatter(string uriString)
    {
        using (XmlReader reader = XmlReader.Create(uriString, settings))
            return CreateItemFormatter(reader);
    }

    /// <summary>
    /// Creates a <see cref="SyndicationItemFormatter"/> according to the 
    /// input format.
    /// </summary>
    /// <param name="uri">Item location</param>
    /// <exception cref="NotSupportedException">The input does not contain a valid RSS 2.0 or Atom 1.0 item.</exception>
    public static SyndicationItemFormatter CreateItemFormatter(Uri uri)
    {
        using (XmlReader reader = XmlReader.Create(uri.ToString(), settings))
            return CreateItemFormatter(reader);
    }

    /// <summary>
    /// Creates a <see cref="SyndicationItemFormatter"/> according to the 
    /// input format.
    /// </summary>
    /// <param name="reader">Item source</param>
    /// <exception cref="NotSupportedException">The input does not contain a valid RSS 2.0 or Atom 1.0 item.</exception>
    public static SyndicationItemFormatter CreateItemFormatter(XmlReader reader)
    {
        if (reader.ReadState == ReadState.Initial)
        {
            reader.MoveToContent();
        }

        Rss20ItemFormatter rss = new Rss20ItemFormatter();
        if (rss.CanRead(reader))
        {
            return rss;
        }

        Atom10ItemFormatter atom = new Atom10ItemFormatter();
        if (atom.CanRead(reader))
        {
            return atom;
        }

        throw new NotSupportedException("Invalid item element: " + reader.Name);
    }
}

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