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Strongly-Typed Extensions Sample

The sample uses the SyndicationFeed class for the purposes of the example. However, the patterns demonstrated in this sample can be used with all of the Syndication classes that support extension data.

The Syndication object model (SyndicationFeed, SyndicationItem, and related classes) supports loosely-typed access to extension data by using the AttributeExtensions and ElementExtensions properties. This sample shows how to provide strongly-typed access to extension data by implementing custom derived classes of SyndicationFeed and SyndicationItem that make available certain application-specific extensions as strongly-typed properties.

As an example, this sample shows how to implement an extension element defined in the proposed Atom Threading Extensions RFC. This is for demonstration purposes only and this sample is not intended to be a full implementation of the proposed specification.

The following XML example shows an Atom 1.0 entry with an additional <in-reply-to> extension element.

<entry>
    <id>tag:example.org,2005:1,2</id>
    <title type="text">Another response to the original</title>
    <summary type="text">
         This is a response to the original entry</summary>
    <updated>2006-03-01T12:12:13Z</updated>
    <link href="http://www.example.org/entries/1/2" />
    <in-reply-to p3:ref="tag:example.org,2005:1" 
                 p3:href="http://www.example.org/entries/1" 
                 p3:type="application/xhtml+xml" 
                 xmlns:p3="http://contoso.org/syndication/thread/1.0" 
                 xmlns="http://contoso.org/syndication/thread/1.0">
      <anotherElement xmlns="http://www.w3.org/2005/Atom">
                     Some more data</anotherElement>
      <aDifferentElement xmlns="http://www.w3.org/2005/Atom">
                     Even more data</aDifferentElement>
    </in-reply-to>
</entry>

The <in-reply-to> element specifies three required attributes (ref, type and href) while also allowing for the presence of additional extension attributes and extension elements.

In this sample, the <in-reply-to> element is modeled as CLR that implements IXmlSerializable, which enables its use with the DataContractSerializer. It also implements some methods and properties for accessing the element’s data, as shown in the following sample code.

[XmlRoot(ElementName = "in-reply-to", Namespace = "http://contoso.org/syndication/thread/1.0")]
public class InReplyToElement : IXmlSerializable
{
    internal const string ElementName = "in-reply-to";
    internal const string NsUri = 
                  "http://contoso.org/syndication/thread/1.0";
    private Dictionary<XmlQualifiedName, string> extensionAttributes;
    private Collection<XElement> extensionElements;

    public InReplyToElement()
    {
        this.extensionElements = new Collection<XElement>();
        this.extensionAttributes = new Dictionary<XmlQualifiedName, 
                                                          string>();
    }

    public Dictionary<XmlQualifiedName, string> AttributeExtensions
    {
        get { return this.extensionAttributes; }
    }

    public Collection<XElement> ElementExtensions
    {
        get { return this.extensionElements; }
    }

    public Uri Href
    { get; set; }

    public string MediaType
    { get; set; }

    public string Ref
    { get; set; }

    public Uri Source
    { get; set; }
}

The InReplyToElement class implements properties for the required attribute (HRef, MediaType, and Source) as well as collections to hold AttributeExtensions and ElementExtensions.

The InReplyToElement class implements the IXmlSerializable interface, which allows direct control over how object instances are read from and written to XML. The ReadXml method first reads the values for the Ref, HRef, Source, and MediaType properties from the XmlReader passed to it. Any unknown attributes are stored in the AttributeExtensions collection. When all the attributes have been read, ReadStartElement() is called to advance the reader to the next element. Because the element modeled by this class has no required children, the child elements get buffered into XElement instances and stored in the ElementExtensions collection, as shown in the following code.

public void ReadXml(System.Xml.XmlReader reader)
{
    bool isEmpty = reader.IsEmptyElement;

    if (reader.HasAttributes)
    {
        for (int i = 0; i < reader.AttributeCount; i++)
        {
            reader.MoveToNextAttribute();

            if (reader.NamespaceURI == "")
            {
                if (reader.LocalName == "ref")
                {
                    this.Ref = reader.Value;
                }
                else if (reader.LocalName == "href")
                {
                    this.Href = new Uri(reader.Value);
                }
                else if (reader.LocalName == "source")
                {
                    this.Source = new Uri(reader.Value);
                }
                else if (reader.LocalName == "type")
                {
                    this.MediaType = reader.Value;
                }
                else
                {
                    this.AttributeExtensions.Add(new 
                                 XmlQualifiedName(reader.LocalName, 
                                 reader.NamespaceURI), 
                                 reader.Value);
                }
            }
        }
    }

    reader.ReadStartElement();

    if (!isEmpty)
    {
        while (reader.IsStartElement())
        {
            ElementExtensions.Add(
                  (XElement) XElement.ReadFrom(reader));
        }
        reader.ReadEndElement();
    }
}

In WriteXml, the InReplyToElement method first writes out the values of the Ref, HRef, Source, and MediaType properties as XML attributes (WriteXml is not responsible for writing the actual outer element itself, as that done by the caller of WriteXml). It also writes the contents of the AttributeExtensions and ElementExtensions to the writer, as shown in the following code.

public void WriteXml(System.Xml.XmlWriter writer)
{
    if (this.Ref != null)
    {
        writer.WriteAttributeString("ref", InReplyToElement.NsUri, 
                                            this.Ref);
    }
    if (this.Href != null)
    {
        writer.WriteAttributeString("href", InReplyToElement.NsUri, 
                                                this.Href.ToString());
    }
    if (this.Source != null)
    {
        writer.WriteAttributeString("source", InReplyToElement.NsUri, 
                                              this.Source.ToString());
    }
    if (this.MediaType != null)
    {
        writer.WriteAttributeString("type", InReplyToElement.NsUri, 
                                                    this.MediaType);
    }

    foreach (KeyValuePair<XmlQualifiedName, string> kvp in 
                                             this.AttributeExtensions)
    {
        writer.WriteAttributeString(kvp.Key.Name, kvp.Key.Namespace, 
                                                   kvp.Value);
    }

    foreach (XElement element in this.ElementExtensions)
    {
        element.WriteTo(writer);
    }
}

In the sample, SyndicationItems with InReplyTo extensions are modeled by the ThreadedItem class. Similarly, the ThreadedFeed class is a SyndicationFeed whose items are all instances of ThreadedItem.

The ThreadedFeed class inherits from SyndicationFeed and overrides OnCreateItem to return a ThreadedItem. It also implements a method for accessing the Items collection as ThreadedItems, as shown in the following code.

public class ThreadedFeed : SyndicationFeed
{
    public ThreadedFeed()
    {
    }

    public IEnumerable<ThreadedItem> ThreadedItems
    {
        get
        {
            return this.Items.Cast<ThreadedItem>();
        }
    }

    protected override SyndicationItem CreateItem()
    {
        return new ThreadedItem();
    }
}

The class ThreadedItem inherits from SyndicationItem and makes InReplyToElement as a strongly-typed property. This provides for convenient programmatic access to the InReplyTo extension data. It also implements TryParseElement and WriteElementExtensions for reading and writing its extension data, as shown in the following code.

public class ThreadedItem : SyndicationItem
{
    private InReplyToElement inReplyTo;
    // Constructors
        public ThreadedItem()
        {
            inReplyTo = new InReplyToElement();
        }

        public ThreadedItem(string title, string content, Uri itemAlternateLink, string id, DateTimeOffset lastUpdatedTime) : base(title, content, itemAlternateLink, id, lastUpdatedTime)
        {
            inReplyTo = new InReplyToElement();
        }

    public InReplyToElement InReplyTo
    {
        get { return this.inReplyTo; }
    }

    protected override bool TryParseElement(
                        System.Xml.XmlReader reader, 
                        string version)
    {
        if (version == SyndicationVersions.Atom10 &&
            reader.NamespaceURI == InReplyToElement.NsUri &&
            reader.LocalName == InReplyToElement.ElementName)
        {
            this.inReplyTo = new InReplyToElement();

            this.InReplyTo.ReadXml(reader);

            return true;
        }
        else
        {
            return base.TryParseElement(reader, version);
        }
    }

    protected override void WriteElementExtensions(XmlWriter writer, 
                                                 string version)
    {
        if (this.InReplyTo != null && 
                     version == SyndicationVersions.Atom10)
        {
            writer.WriteStartElement(InReplyToElement.ElementName, 
                                           InReplyToElement.NsUri);
            this.InReplyTo.WriteXml(writer);
            writer.WriteEndElement();
        }

        base.WriteElementExtensions(writer, version);
    }
}

To set up, build, and run the sample

  1. Ensure that you have performed the One-Time Setup Procedure for the Windows Communication Foundation Samples.

  2. To build the C# or Visual Basic .NET edition of the solution, follow the instructions in Building the Windows Communication Foundation Samples.

  3. To run the sample in a single- or cross-machine configuration, follow the instructions in Running the Windows Communication Foundation Samples.

Important note Important

The samples may already be installed on your computer. Check for the following (default) directory before continuing.

<InstallDrive>:\WF_WCF_Samples

If this directory does not exist, go to Windows Communication Foundation (WCF) and Windows Workflow Foundation (WF) Samples for .NET Framework 4 to download all Windows Communication Foundation (WCF) and WF samples. This sample is located in the following directory.

<InstallDrive>:\WF_WCF_Samples\WCF\Extensibility\Syndication\StronglyTypedExtensions

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