从 WCF 客户端内部调试 NullReference 异常

问：

我们有一个 WCF 服务和大致如下的客户端代码：

bool success = false;
IClientChannel proxy = null;
try
{
  var client = channelFactory.CreateChannel(new EndpointAddress(url));
  proxy = client as IClientChannel;
  proxy.Open();
  client.MyWcfMethod(a, b, c);                                
  proxy.Close();
  success = true;
}
finally
{
  if (!success)
    proxy.Abort();
}

它似乎工作正常，但是在我们的测试环境中，~70000 次调用中会抛出一次 NullReferenceException。我们仔细记录异常和周围的所有数据，看起来它不以任何方式依赖于数据。如果我在异常后立即重复调用（距离成功只有一段时间），它会顺利进行。

该异常似乎非常罕见，以至于我无法重现、跟踪或调试它。调用时似乎立即抛出异常，没有调用服务器代码。

下面是堆栈跟踪：

Object reference not set to an instance of an object. Server stack trace: 
at System.Xml.Linq.ElementWriter.WriteElement(XElement e) 
at System.Xml.Linq.XElement.WriteTo(XmlWriter writer) 
at System.Xml.Linq.XElement.System.Xml.Serialization.IXmlSerializable.WriteXml(XmlWriter writer) 
at System.Runtime.Serialization.XmlObjectSerializerWriteContext.WriteIXmlSerializable(XmlWriterDelegator xmlWriter, Object obj, XmlSerializableWriter xmlSerializableWriter) 
at System.Runtime.Serialization.XmlObjectSerializerWriteContext.WriteIXmlSerializable(XmlWriterDelegator xmlWriter, Object obj) 
at System.Runtime.Serialization.XmlDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context) 
at System.Runtime.Serialization.XmlObjectSerializerWriteContext.WriteDataContractValue(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle) 
at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithoutXsiType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle) 
at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle) 
at WriteCallContextToXml(XmlWriterDelegator , Object , XmlObjectSerializerWriteContext , ClassDataContract ) 
at System.Runtime.Serialization.ClassDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context) 
at System.Runtime.Serialization.XmlObjectSerializerWriteContext.WriteDataContractValue(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle) 
at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithoutXsiType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle) 
at System.Runtime.Serialization.DataContractSerializer.InternalWriteObjectContent(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver) 
at System.Runtime.Serialization.DataContractSerializer.InternalWriteObjectContent(XmlWriterDelegator writer, Object graph) 
at System.Runtime.Serialization.XmlObjectSerializer.WriteObjectContentHandleExceptions(XmlWriterDelegator writer, Object graph) 
at System.Runtime.Serialization.DataContractSerializer.WriteObjectContent(XmlDictionaryWriter writer, Object graph) 
at System.ServiceModel.Channels.XmlObjectSerializerHeader.OnWriteHeaderContents(XmlDictionaryWriter writer, MessageVersion messageVersion) 
at System.ServiceModel.Channels.MessageHeader.WriteHeaderContents(XmlDictionaryWriter writer, MessageVersion messageVersion) 
at System.ServiceModel.Channels.MessageHeaders.WriteHeaderContents(Int32 headerIndex, XmlDictionaryWriter writer) 
at System.ServiceModel.Channels.Message.WriteMessagePreamble(XmlDictionaryWriter writer) at System.ServiceModel.Channels.Message.OnWriteMessage(XmlDictionaryWriter writer) at System.ServiceModel.Channels.BufferedMessageWriter.WriteMessage(Message message, BufferManager bufferManager, Int32 initialOffset, Int32 maxSizeQuota) 
at System.ServiceModel.Channels.BinaryMessageEncoderFactory.BinaryMessageEncoder.WriteMessage(Message message, Int32 maxMessageSize, BufferManager bufferManager, Int32 messageOffset) 
at System.ServiceModel.Channels.HttpOutput.SerializeBufferedMessage(Message message) 
at System.ServiceModel.Channels.HttpOutput.Send(TimeSpan timeout) 
at System.ServiceModel.Channels.HttpChannelFactory`1.HttpRequestChannel.HttpChannelRequest.SendRequest(Message message, TimeSpan timeout) 
at System.ServiceModel.Channels.RequestChannel.Request(Message message, TimeSpan timeout) at System.ServiceModel.Dispatcher.RequestChannelBinder.Request(Message message, TimeSpan timeout) at System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs, TimeSpan timeout) 
at System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(IMethodCallMessage methodCall, ProxyOperationRuntime operation) 
at System.ServiceModel.Channels.ServiceChannelProxy.Invoke(IMessage message) Exception rethrown at [0]: at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg) 
at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type) 
at IMyService.MyWcfMethod(int a, string b, double c) 
at MyClientClass.Process() 

关于在哪里寻找答案的任何想法？