Using the XmlSerializer as an XmlObjectSerializer with WCF

The Windows Communication Foundation introduced the new DataContractSerializer to replace the XmlSerializer in many scenarios.  The DataContractSerializer is faster than the XmlSerializer, but has certain limitations.  For example it does not support serializing an object's members as XML attributes.  When you write a WCF client that calls a service, WCF will generate a proxy class that defaults to using the DataContractSerializer and falls back to the XmlSerializer if the service uses features that the DataContractSerializer does not support.  This fallback mechanism is automatic if you're using WCF's service model.

But if you're working at WCF's messaging layer, you must explicitly choose which serializer to use.  You can use any serializer that derives from System.Runtime.Serialization.XmlObjectSerializer.  System.Runtime.Serialization.DataContractSerializer already derives from this class -- but System.Xml.Serialization.XmlSerializer does not.  So you have some work to do before you can use the XmlSerializer as your serializer of choice at the messaging layer of WCF.

In order to use the XmlSerializer to send messages with WCF, we have to define a class that derives from XmlObjectSerializer and uses the XmlSerializer internally.  In essence we will create a wrapper class for the XmlSerializer that makes it look and work like an XmlObjectSerializer.  XmlObjectSerializer is an abstract class with only a few methods we need to implement. 

Below is a sample of a class that will do just that.

namespace Microsoft.ServiceModel.Samples
{
    using System;
    using System.Diagnostics;
    using System.IO;
    using System.Text;
    using System.Xml;
    using System.Xml.Serialization;
    using System.Runtime.Serialization;

    public class CFMessagingSerializer : XmlObjectSerializer
    {
        readonly Type objectType;
        XmlSerializer serializer;

        public CFMessagingSerializer(Type objectType)
            : this(objectType, null, null)
        {
        }
        
        public CFMessagingSerializer(Type objectType, string wrapperName, string wrapperNamespace)
        {
            if (objectType == null)
                throw new ArgumentNullException("objectType");
            if ((wrapperName == null) != (wrapperNamespace == null))
                throw new ArgumentException("wrapperName and wrapperNamespace must be either both null or both non-null.");
            if (wrapperName == string.Empty)
                throw new ArgumentException("Cannot be the empty string.", "wrapperName");
            
            this.objectType = objectType;
            if (wrapperName != null)
            {
                XmlRootAttribute root = new XmlRootAttribute(wrapperName);
                root.Namespace = wrapperNamespace;
                this.serializer = new XmlSerializer(objectType, root);
            }
            else
                this.serializer = new XmlSerializer(objectType);
        }

        public override bool IsStartObject(XmlDictionaryReader reader)
        {
            throw new NotImplementedException();
        }

        public override object ReadObject(XmlDictionaryReader reader, bool verifyObjectName)
        {
            Debug.Assert(serializer != null);
            if (reader == null) throw new ArgumentNullException("reader");
            if (!verifyObjectName)
                throw new NotSupportedException();

            return serializer.Deserialize(reader);
        }
        
        public override void WriteStartObject(XmlDictionaryWriter writer, object graph)
        {
            throw new NotImplementedException();
        }

        public override void WriteObjectContent(XmlDictionaryWriter writer, object graph)
        {
            if (writer == null) throw new ArgumentNullException("writer");
            if (writer.WriteState != WriteState.Element)
                throw new SerializationException(string.Format("WriteState '{0}' not valid. Caller must write start element before serializing in contentOnly mode.",
                    writer.WriteState));
            using (MemoryStream memoryStream = new MemoryStream())
            {
                using (XmlDictionaryWriter bufferWriter = XmlDictionaryWriter.CreateTextWriter(memoryStream, Encoding.UTF8))
                {
                    serializer.Serialize(bufferWriter, graph);
                    bufferWriter.Flush();
                    memoryStream.Position = 0;
                    using (XmlReader reader = new XmlTextReader(memoryStream))
                    {
                        reader.MoveToContent();
                        writer.WriteAttributes(reader, false);
                        if (reader.Read()) // move off start node (we want to skip it)
                        {
                            while (reader.NodeType != XmlNodeType.EndElement) // also skip end node.
                                writer.WriteNode(reader, false); // this will take us to the start of the next child node, or the end node.
                            reader.ReadEndElement(); // not necessary, but clean
                        }
                    }
                }
            }
        }

        public override void WriteEndObject(XmlDictionaryWriter writer)
        {
            throw new NotImplementedException();
        }
        
        public override void WriteObject(XmlDictionaryWriter writer, object graph)
        {
            Debug.Assert(serializer != null);
            if (writer == null) throw new ArgumentNullException("writer");
            serializer.Serialize(writer, graph);
        }
    }
}