.net 用soap扩展类把 soap消息记录到日志文件

.net 用soap扩展类把 soap消息记录到日志文件中，请求soap消息可以记录到日志文件，但是在响应消息出现错误：
System.Web.Services.Protocols.SoapException: 服务器无法处理请求。 ---> System.NotSupportedException: 不支持所指定的方法。
   在 System.Web.HttpResponseStream.Read(Byte[] buffer, Int32 offset, Int32 count)
   在 System.Web.Services.Protocols.SoapExtensionStream.Read(Byte[] buffer, Int32 offset, Int32 count)
   在 System.Xml.XmlTextReaderImpl.InitStreamInput(Uri baseUri, String baseUriStr, Stream stream, Byte[] bytes, Int32 byteCount, Encoding encoding)
   在 System.Xml.XmlTextReaderImpl..ctor(String url, Stream input, XmlNameTable nt)
   在 System.Xml.XmlDocument.Load(Stream inStream)
   在 eHealthSer.TraceExtension.WriteOutput(SoapMessage message) 位置 C:\workspace\eHealthSer\eHealthSer\security\TraceExtension.cs:行号 132
   在 eHealthSer.TraceExtension.ProcessMessage(SoapMessage message) 位置 C:\workspace\eHealthSer\eHealthSer\security\TraceExtension.cs:行号 65
   在 System.Web.Services.Protocols.SoapMessage.RunExtensions(SoapExtension[] extensions, Boolean throwOnException)
   在 System.Web.Services.Protocols.SoapServerProtocol.WriteReturns(Object[] returnValues, Stream outputStream)
   在 System.Web.Services.Protocols.WebServiceHandler.WriteReturns(Object[] returnValues)
   在 System.Web.Services.Protocols.WebServiceHandler.Invoke()
   --- 内部异常堆栈跟踪的结尾 ---

soap 扩展类如下：
  public class TraceExtension :SoapExtension
    {
        log4net.ILog logger;
        Stream httpStream;
         public override object GetInitializer(Type serviceType)
        {
            return null;
        }
        public override object GetInitializer(LogicalMethodInfo methodInfo, SoapExtensionAttribute attribute)
        {
            return null;
        }

         public override void Initialize(object initializer)
        {
            logger = log4net.LogManager.GetLogger("Debug");
            logger.Debug("Initialize:" + initializer);
        }
        public override System.IO.Stream ChainStream(System.IO.Stream stream)
        {
            httpStream = stream;
            return httpStream;
        }

        public override void ProcessMessage(SoapMessage message)
        {
            switch (message.Stage)
            {
                //object --xml 对象解析成XML格式之后的阶段
                case SoapMessageStage.AfterSerialize:
                    WriteOutput(message);
                    break;
                //object --xml 对象解析成XML格式之前的阶段
                case SoapMessageStage.BeforeSerialize:
                    test(message);
                    break;
                //xml -- object XML 格式解析成对象之前的阶段
                case SoapMessageStage.BeforeDeserialize:
                    WriteInput(message);
                    break;
                case SoapMessageStage.AfterDeserialize:

                    break;
                default:
                    break;
            }
        }

         private void WriteInput(SoapMessage message)
        {
            if (message is SoapServerMessage)
            {
                XmlDocument doc = new XmlDocument();
                long posStream = httpStream.Position;
                //httpStream = message.Stream;
                doc.Load(httpStream);
                httpStream.Position = posStream;
                logger.Debug("------------Input SoapMessage--------------");
                //  logger.Debug("Headers:"+doc.);
                logger.Debug("PlayLoad:" + doc.InnerXml);
               // httpStream.Flush();
            }
         }
           private void WriteOutput(SoapMessage message)
        {
            if (message is SoapServerMessage)
            {
                 XmlDocument doc = new XmlDocument();
                 doc.Load(httpStream);
             logger.Debug("------------Output SoapMessage------------");
                logger.Debug("PlayLoad:");
            }
}

--------------------编程问答-------------------- 有没有谁知道，帮帮忙，困恼了很久了

补充：.NET技术 ,  ASP.NET