Snippet to create and read a System.ServiceModel.Channels.Message object representing a WCF operation invocation containing multiple parameters. This is how you would use DataContractSerializer to make an entire operation call, including each individual parameter, rather than just serializing one object at a time.

SoapUtilities.cs

using System;
using System.IO;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel.Channels;
using System.ServiceModel.Description;
using System.Text;
using System.Xml;

public static class SoapUtilities
{
    class RequestMessageBodyWriter : BodyWriter
    {
        private MessageBodyDescription bodyDescription;
        private object[] parameters;

        public RequestMessageBodyWriter(MessageBodyDescription bodyDescription, object[] parameters)
            : base(false)
        {
            this.bodyDescription = bodyDescription;
            this.parameters = parameters;
        }

        protected override void OnWriteBodyContents(XmlDictionaryWriter writer)
        {
            // Can't Dispose this xmlWriter, 'cause it will close 'writer'.
            var xmlWriter = XmlWriter.Create(writer);

            xmlWriter.WriteStartElement(bodyDescription.WrapperName, bodyDescription.WrapperNamespace);
            foreach (var part in bodyDescription.Parts)
            {
                var dcs = new DataContractSerializer(part.Type, part.Name, part.Namespace);
                dcs.WriteObject(xmlWriter, parameters[part.Index]);
            }
            xmlWriter.WriteEndElement();
        }
    }

    /// <summary>
    /// Creates a SOAP message representing an operation call.
    /// </summary>
    /// <param name="parameters">An array of objects representing the parameters of the operation.</param>
    /// <param name="serviceContractType">The Type object of the service contract definition.</param>
    /// <param name="operationContractName">The name of the operation method.</param>
    public static string WriteOperationRequest(object[] parameters, Type serviceContractType, string operationContractName)
    {
        var serviceDescription = ContractDescription.GetContract(serviceContractType);
        var operationDescription = serviceDescription.Operations.Single(op => op.Name == operationContractName);
        var inputMessage = operationDescription.Messages.Single(msgDescription => msgDescription.Direction == MessageDirection.Input);
        var body = inputMessage.Body;

        var soapMessage = Message.CreateMessage(
            MessageVersion.Soap12,
            serviceDescription.Namespace + serviceDescription.Name + "/" + operationDescription.Name,
            new RequestMessageBodyWriter(body, parameters)
        );

        var sb = new StringBuilder();
        using (var xmlWriter = XmlWriter.Create(sb))
            soapMessage.WriteMessage(xmlWriter);
        return sb.ToString();
    }

    /// <summary>
    /// Reads a SOAP message for a specific operation and deserializes the operation parameters.  This is the 
    /// inverse of WriteOperationRequest.
    /// </summary>
    /// <param name="message">The message content.</param>
    /// <param name="serviceContractType">The Type object of the service contract definition.</param>
    /// <param name="operationContractName">The name of the operation method.</param>
    public static object[] ReadOperationParameters(string message, Type serviceContractType, string operationContractName)
    {
        var serviceDescription = ContractDescription.GetContract(serviceContractType);
        var operationDescription = serviceDescription.Operations.Single(op => op.Name == operationContractName);
        var inputMessage = operationDescription.Messages.Single(msgDescription => msgDescription.Direction == MessageDirection.Input);
        var bodyDescription = inputMessage.Body;

        object[] parameters = new object[inputMessage.Body.Parts.Count];

        using (var stringReader = new StringReader(message))
        using (var messageReader = XmlReader.Create(stringReader))
        {
            var soapMessage = System.ServiceModel.Channels.Message.CreateMessage(
                messageReader,
                1024 * 1024,
                System.ServiceModel.Channels.MessageVersion.Soap12
            );

            var bodyReader = soapMessage.GetReaderAtBodyContents();
            bodyReader.ReadStartElement(bodyDescription.WrapperName, bodyDescription.WrapperNamespace);
            foreach (var part in bodyDescription.Parts)
            {
                var dcs = new DataContractSerializer(part.Type, part.Name, part.Namespace);
                parameters[part.Index] = dcs.ReadObject(bodyReader);
            }
            bodyReader.ReadEndElement();
        }

        return parameters;
    }
}