WCF Service Reference - Getting "XmlException: Name cannot begin with the '<' character, hexadecimal value 0x3C" on Client Side

Question

I have a smart client application communicating with its server via WCF. Data is created on the client and then sent over the service to be persisted. The server and client use the same domain classes via a shared dll and I'm using the handy "Add Service Reference" functionality in Visual Studio that wraps SvcUtil.exe and generates the client and proxy classes.

I get the following error when trying to call the service:

System.Xml.XmlException occurred   Message=Name cannot begin with the '<' character, hexadecimal value 0x3C.   Source=System.Xml   LineNumber=0   LinePosition=1   StackTrace:        at System.Xml.XmlConvert.VerifyNCName(String name, ExceptionType exceptionType)   InnerException:  

This is particularly troublesome because the service will work for weeks at a time without this error occurring and then suddenly and without warning it will appear again. I haven't been able to figure out what causes it at all. When it does happen, I will do a deep dive into researching how to fix it and usually don't come up with anything more than people who have experienced the same error while actually trying to serialize things to xml programmatically. I am using nothing but the generated client and proxies to try and send this data.

I have looked into the generated proxies in the Service References\AwesomeService folder of my solution and see nothing that looks out of the ordinary. The only occurrences of angle brackets in the generated files are:

• xml tags in the *.svcinfo, Reference.svcmap, AwesomeService.disco, AwesomeService.wsdl and *.xsd files
• arguments to generic collections in the Reference.cs file

The code I'm using to call the service is such:

using (var client = new AwesomeServiceClient()) {     client.SaveAwesomeness(instanceOfAwesomeness); } 

This is the stack beggining with the first frame above the calling code listed above:

System.Xml.dll!System.Xml.XmlConvert.VerifyNCName(string name, System.Xml.ExceptionType exceptionType) + 0xb5 bytes  System.Runtime.Serialization.dll!System.Runtime.Serialization.DataContract.IsValidNCName(string name) + 0x27 bytes   System.Runtime.Serialization.dll!System.Runtime.Serialization.DataContract.EncodeLocalName(string localName) + 0x1d bytes    System.Runtime.Serialization.dll!System.Runtime.Serialization.ClassDataContract.ClassDataContractCriticalHelper.ImportDataMembers() + 0x2e1 bytes    System.Runtime.Serialization.dll!System.Runtime.Serialization.ClassDataContract.ClassDataContractCriticalHelper.ClassDataContractCriticalHelper(System.Type type) + 0x10d bytes  System.Runtime.Serialization.dll!System.Runtime.Serialization.DataContract.DataContractCriticalHelper.CreateDataContract(int id, System.RuntimeTypeHandle typeHandle, System.Type type) + 0x198 bytes    System.Runtime.Serialization.dll!System.Runtime.Serialization.DataContract.DataContractCriticalHelper.GetDataContractSkipValidation(int id, System.RuntimeTypeHandle typeHandle, System.Type type) + 0x57 bytes  System.Runtime.Serialization.dll!System.Runtime.Serialization.XmlObjectSerializerContext.GetDataContract(int id, System.RuntimeTypeHandle typeHandle) + 0x37 bytes   System.Runtime.Serialization.dll!System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerialize(System.Runtime.Serialization.XmlWriterDelegator xmlWriter, object obj, bool isDeclaredType, bool writeXsiType, int declaredTypeID, System.RuntimeTypeHandle declaredTypeHandle) + 0x49 bytes     [Lightweight Function]   System.Runtime.Serialization.dll!System.Runtime.Serialization.ClassDataContract.WriteXmlValue(System.Runtime.Serialization.XmlWriterDelegator xmlWriter, object obj, System.Runtime.Serialization.XmlObjectSerializerWriteContext context) + 0x25 bytes  System.Runtime.Serialization.dll!System.Runtime.Serialization.XmlObjectSerializerWriteContext.WriteDataContractValue(System.Runtime.Serialization.DataContract dataContract, System.Runtime.Serialization.XmlWriterDelegator xmlWriter, object obj, System.RuntimeTypeHandle declaredTypeHandle) + 0x18 bytes    System.Runtime.Serialization.dll!System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithoutXsiType(System.Runtime.Serialization.DataContract dataContract, System.Runtime.Serialization.XmlWriterDelegator xmlWriter, object obj, System.RuntimeTypeHandle declaredTypeHandle) + 0x49 bytes   System.Runtime.Serialization.dll!System.Runtime.Serialization.DataContractSerializer.InternalWriteObjectContent(System.Runtime.Serialization.XmlWriterDelegator writer, object graph, System.Runtime.Serialization.DataContractResolver dataContractResolver) + 0xdf bytes   System.Runtime.Serialization.dll!System.Runtime.Serialization.DataContractSerializer.InternalWriteObject(System.Runtime.Serialization.XmlWriterDelegator writer, object graph, System.Runtime.Serialization.DataContractResolver dataContractResolver) + 0x26 bytes  System.Runtime.Serialization.dll!System.Runtime.Serialization.XmlObjectSerializer.WriteObjectHandleExceptions(System.Runtime.Serialization.XmlWriterDelegator writer, object graph, System.Runtime.Serialization.DataContractResolver dataContractResolver) + 0x60 bytes     System.Runtime.Serialization.dll!System.Runtime.Serialization.XmlObjectSerializer.WriteObject(System.Xml.XmlDictionaryWriter writer, object graph) + 0x2d bytes  System.ServiceModel.dll!System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeParameterPart(System.Xml.XmlDictionaryWriter writer, System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.PartInfo part, object graph) + 0x38 bytes  System.ServiceModel.dll!System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeParameter(System.Xml.XmlDictionaryWriter writer, System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.PartInfo part, object graph) + 0xbe bytes  System.ServiceModel.dll!System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeParameters(System.Xml.XmlDictionaryWriter writer, System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.PartInfo[] parts, object[] parameters) + 0x3e bytes   System.ServiceModel.dll!System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeBody(System.Xml.XmlDictionaryWriter writer, System.ServiceModel.Channels.MessageVersion version, string action, System.ServiceModel.Description.MessageDescription messageDescription, object returnValue, object[] parameters, bool isRequest) + 0x68 bytes    System.ServiceModel.dll!System.ServiceModel.Dispatcher.OperationFormatter.SerializeBodyContents(System.Xml.XmlDictionaryWriter writer, System.ServiceModel.Channels.MessageVersion version, object[] parameters, object returnValue, bool isRequest) + 0x7b bytes    System.ServiceModel.dll!System.ServiceModel.Dispatcher.OperationFormatter.OperationFormatterMessage.OperationFormatterBodyWriter.OnWriteBodyContents(System.Xml.XmlDictionaryWriter writer) + 0x4f bytes     System.ServiceModel.dll!System.ServiceModel.Channels.BodyWriter.WriteBodyContents(System.Xml.XmlDictionaryWriter writer) + 0xf8 bytes    System.ServiceModel.dll!System.ServiceModel.Channels.BodyWriterMessage.OnBodyToString(System.Xml.XmlDictionaryWriter writer) + 0x1f bytes    System.ServiceModel.dll!System.ServiceModel.Channels.Message.ToString(System.Xml.XmlDictionaryWriter writer) + 0xaa bytes    System.ServiceModel.dll!System.ServiceModel.Diagnostics.MessageLogTraceRecord.WriteTo(System.Xml.XmlWriter writer) + 0x166 bytes     System.ServiceModel.dll!System.ServiceModel.Diagnostics.MessageLogger.LogInternal(System.ServiceModel.Diagnostics.MessageLogTraceRecord record) + 0x77 bytes     System.ServiceModel.dll!System.ServiceModel.Diagnostics.MessageLogger.LogMessageImpl(ref System.ServiceModel.Channels.Message message, System.Xml.XmlReader reader, System.ServiceModel.Diagnostics.MessageLoggingSource source) + 0x104 bytes   System.ServiceModel.dll!System.ServiceModel.Diagnostics.MessageLogger.LogMessage(ref System.ServiceModel.Channels.Message message, System.Xml.XmlReader reader, System.ServiceModel.Diagnostics.MessageLoggingSource source) + 0x3a bytes    System.ServiceModel.dll!System.ServiceModel.Channels.ServiceChannel.PrepareCall(System.ServiceModel.Dispatcher.ProxyOperationRuntime operation, bool oneway, ref System.ServiceModel.Dispatcher.ProxyRpc rpc) + 0x436 bytes  System.ServiceModel.dll!System.ServiceModel.Channels.ServiceChannel.Call(string action, bool oneway, System.ServiceModel.Dispatcher.ProxyOperationRuntime operation, object[] ins, object[] outs, System.TimeSpan timeout) + 0x12b bytes     System.ServiceModel.dll!System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(System.Runtime.Remoting.Messaging.IMethodCallMessage methodCall, System.ServiceModel.Dispatcher.ProxyOperationRuntime operation) + 0x64 bytes     System.ServiceModel.dll!System.ServiceModel.Channels.ServiceChannelProxy.Invoke(System.Runtime.Remoting.Messaging.IMessage message) + 0x6a bytes     mscorlib.dll!System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(ref System.Runtime.Remoting.Proxies.MessageData msgData, int type) + 0xee bytes 

What causes this and how do I prevent it? Or, also welcomed, how do I go about troubleshooting this further?

Answer 1

Have a look at your DataTables (if that's what you are using to transport data).

If the DataTable name is empty, then the Serializer might get confused and serialise things incorrectly.

Otherwise, if you are using typed, [Serializable] objects, I have found that sometimes the Serializer also gets confused if you use dynamic property declarations, eg:

public string MyName { get; set; } 

But this would be a easily repeatable error.

Answer 2

Personnally, I got the same problem with serialization of class hierarchy (not DataTables).

My problem was not related to automatic property at all, in fact I have many. My problem was that I forgot to include reference to "System.Runtime.Serialization" in one of my dll and I also forgot to add some attributes [DataContract] on some classes referenced by upper [DataMember] attributes up in the hierarchy.

To track my problem I started from my root class and removed some [DataMember] down the hierarchy until it pointed out the exact problem. It could takes some times depending on your hierarchy levels...

Hope it helps! Eric