WCF 的 DataContractSerilaizer 线程安全吗？

Question

我一直在将一个相当大的系统从 Remoting 转换为 WCF，一切似乎都运行良好，除了我们经常遇到以下异常：“System.InvalidOperationException：集合已修改；枚举操作可能无法执行。”我没有任何运气来追踪它，因为它只在有数百个调用通过时才会发生，我只能假设这是因为一个对象在被序列化时正在被修改。

所有类都使用：[DataContract(IsReference=true)]。

使用远程处理时没有类似的异常，所以我想知道是否有人在 WCF 中遇到过类似的问题，或者可以让我知道它可能是序列化程序——在这种情况下，我假设我必须自己编写序列化程序在必要时执行locks（这是我希望避免的一项重大任务）。

以下是堆栈跟踪：

WCF Error: at System.Collections.Generic.List1.Enumerator.MoveNextRare() at WriteArrayOfLineToXml(XmlWriterDelegator , Object , XmlObjectSerializerWriteContext , CollectionDataContract ) at System.Runtime.Serialization.CollectionDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerializeReference(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle) at WriteLineGroupToXml(XmlWriterDelegator , Object , XmlObjectSerializerWriteContext , ClassDataContract ) at System.Runtime.Serialization.ClassDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context) 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 System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerializeReference(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle) at WriteLineToXml(XmlWriterDelegator , Object , XmlObjectSerializerWriteContext , ClassDataContract ) at System.Runtime.Serialization.ClassDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeAndVerifyType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, Boolean verifyKnownType, RuntimeTypeHandle declaredTypeHandle, Type declaredType) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithXsiTypeAtTopLevel(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle originalDeclaredTypeHandle, Type graphType) at System.Runtime.Serialization.DataContractSerializer.InternalWriteObjectContent(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver) at System.Runtime.Serialization.DataContractSerializer.InternalWriteObject(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver) at System.Runtime.Serialization.XmlObjectSerializer.WriteObjectHandleExceptions(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver) at System.Runtime.Serialization.XmlObjectSerializer.WriteObject(XmlDictionaryWriter writer, Object graph) at System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeParameterPart(XmlDictionaryWriter writer, PartInfo part, Object graph) at System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeParameter(XmlDictionaryWriter writer, PartInfo part, Object graph) at System.ServiceModel.Dispatcher.DataContractSerializerOperationFormatter.SerializeBody(XmlDictionaryWriter writer, MessageVersion version, String action, MessageDescription messageDescription, Object returnValue, Object[] parameters, Boolean isRequest) at System.ServiceModel.Dispatcher.OperationFormatter.SerializeBodyContents(XmlDictionaryWriter writer, MessageVersion version, Object[] parameters, Object returnValue, Boolean isRequest) at System.ServiceModel.Dispatcher.OperationFormatter.OperationFormatterMessage.OperationFormatterBodyWriter.OnWriteBodyContents(XmlDictionaryWriter writer) at System.ServiceModel.Channels.BodyWriter.WriteBodyContents(XmlDictionaryWriter writer) at System.ServiceModel.Channels.BodyWriterMessage.OnWriteBodyContents(XmlDictionaryWriter writer) at System.ServiceModel.Channels.Message.OnWriteMessage(XmlDictionaryWriter writer) at System.ServiceModel.Channels.Message.WriteMessage(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.FramingDuplexSessionChannel.EncodeMessage(Message message) at System.ServiceModel.Channels.FramingDuplexSessionChannel.OnSend(Message message, TimeSpan timeout) at System.ServiceModel.Channels.OutputChannel.Send(Message message, TimeSpan timeout) at System.ServiceModel.Dispatcher.DuplexChannelBinder.DuplexRequestContext.OnReply(Message message, TimeSpan timeout) at System.ServiceModel.Channels.RequestContextBase.Reply(Message message, TimeSpan timeout) at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.Reply(MessageRpc& rpc)

Answer 1

确实，使用DataContractSerializer 可以轻松重现此错误。它不是指DataContractSerializer 的线程安全，它是关于某些集合的线程安全，用于您的数据合约：

[DataContract]
public class C
{
    [DataMember]
    public List<int> Values { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        var c = new C
        {
            Values = new List<int>()
        };

        var serializer = new DataContractSerializer(typeof(C));

        Task
            .Factory
            .StartNew(() => 
            {
                while (true)
                {
                    Console.WriteLine("Trying to add new item.");
                    c.Values.Add(DateTime.Now.Millisecond);
                }
            }, 
            TaskCreationOptions.LongRunning);

        Task
            .Factory
            .StartNew(() =>
            {
                while (true)
                {
                    using (var stream = new MemoryStream())
                    {
                        Console.WriteLine("Trying to serialize.");
                        serializer.WriteObject(stream, c);
                    }
                }
            },
            TaskCreationOptions.LongRunning);

        Console.ReadLine();
    }

在很短的执行时间之后，您将获得具有类似堆栈跟踪的 IOE：

   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)    at System.Collections.Generic.List`1.Enumerator.MoveNextRare()    at System.Collections.Generic.List`1.Enumerator.MoveNext()    at WriteArrayOfintToXml(XmlWriterDelegator , Object , XmlObjectSerializerWriteContext , CollectionDataContract )    at System.Runtime.Serialization.CollectionDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context)

看起来您正在继续修改一些共享数据，这些数据同时被序列化。您可以打开 WCF 跟踪（请参阅this 问题）以找出导致此错误的操作，并仔细查看此操作正在使用哪些数据。

然后，根据当前服务行为的InstanceContextMode和ConcurrencyMode属性的值，可以选择走哪条路：

• 要么使用某种锁定；
• 或使用任何线程安全的集合；
• 或改变服务行为；
• 或更改服务本身（例如，使其无状态）。

Answer 2

如果丹尼斯的假设是正确的，那么解决这个问题的最简单方法是复制集合并通过网络发送副本。此时，在序列化期间是否修改原始内容并不重要