Implementing WCF method that supports either string or object as a method parameter

I have the following contract in a WCF-based web service:

public List<string> GetAllPossibleQueryEngineHostNames(Instance instance);

Obviously, I can call this and pass in an Instance object. However, I need to also support older versions of the client, which pass in a string like so:

string instance = "value";
svc.GetAllPossibleQueryEngineHostNames(instance);

To do this, I created a WCF behavior. This was new to me, which is why I'd appreciate some code review feedback. Note, right now only de-serialization is supported. Don't worry that WriteObjectContent is not implemented. I'll do that later, if I have the need.

public class InstanceSerializer : XmlObjectSerializer
{
    const string localName = "instance";

    public override bool IsStartObject(XmlDictionaryReader reader)
    {
        return String.Equals(reader.LocalName, localName, StringComparison.OrdinalIgnoreCase);
    }

    public override object ReadObject(XmlDictionaryReader reader, bool verifyObjectName)
    {
        string xml = reader.ReadOuterXml();
        XDocument doc = XDocument.Parse(xml);

        string shortCode = doc.Descendants()
            .Where(e => e.Name.LocalName == "ShortCode")
            .Select(e => e.Value)
            .FirstOrDefault();

        string connStr = doc.Descendants()
            .Where(e => e.Name.LocalName == "ConnectionString")
            .Select(e => e.Value)
            .FirstOrDefault();

        if (connStr != null || shortCode != null) // Instance passed as Instance object
        {
            return new Instance(shortCode, connStr);
        }

        // Instance passed as String
        Instance instance = ((XElement) doc.FirstNode).Value;
        return instance;
    }

    public override void WriteEndObject(XmlDictionaryWriter writer)
    {
        writer.WriteEndElement();
    }

    public override void WriteObjectContent(XmlDictionaryWriter writer, object graph)
    {
    }

    public override void WriteStartObject(XmlDictionaryWriter writer, object graph)
    {
        writer.WriteStartElement(localName);
    }
}

public class InstanceBehavior : DataContractSerializerOperationBehavior
{
    public InstanceBehavior(OperationDescription operation) : base(operation) { }

    public override XmlObjectSerializer CreateSerializer(Type type, string name, string ns, IList<Type> knownTypes)
    {
        return typeof(Instance) == type
            ? new InstanceSerializer()
            : base.CreateSerializer(type, name, ns, knownTypes);
    }

    public override XmlObjectSerializer CreateSerializer(Type type, XmlDictionaryString name, XmlDictionaryString ns, IList<Type> knownTypes)
    {
        return typeof(Instance) == type
            ? new InstanceSerializer()
            : base.CreateSerializer(type, name, ns, knownTypes);
    }
}

public class SupportStringInstanceAttribute : Attribute, IContractBehavior
{
    public void AddBindingParameters(ContractDescription contractDescription, ServiceEndpoint endpoint, System.ServiceModel.Channels.BindingParameterCollection bindingParameters)
    {
    }

    public void ApplyClientBehavior(ContractDescription contractDescription, ServiceEndpoint endpoint, System.ServiceModel.Dispatcher.ClientRuntime clientRuntime)
    {
        ReplaceSerializerOperationBehavior(contractDescription);
    }

    public void ApplyDispatchBehavior(ContractDescription contractDescription, ServiceEndpoint endpoint, System.ServiceModel.Dispatcher.DispatchRuntime dispatchRuntime)
    {
        ReplaceSerializerOperationBehavior(contractDescription);
    }

    public void Validate(ContractDescription contractDescription, ServiceEndpoint endpoint)
    {
    }

    private static void ReplaceSerializerOperationBehavior(ContractDescription contract)
    {
        foreach (OperationDescription od in contract.Operations)
        {
            for (int i = 0; i < od.Behaviors.Count; i++)
            {
                DataContractSerializerOperationBehavior dcsob = od.Behaviors[i] as DataContractSerializerOperationBehavior;
                if (dcsob != null)
                {
                    od.Behaviors[i] = new InstanceBehavior(od);
                }
            }
        }
    }
}