由“yield return”语句支持的 IEnumerable<> 的 DataContract 序列化

Question

是否可以序列化 IEnumerable 属性，其中值由“yield return”语句支持？如果可能，怎么做？不是，为什么？

每当我尝试这样做时，都会从 DataContractSerializer 获得 NullReferenceException。一个例子：

using System.Collections.Generic;
using System.IO;
using System.Runtime.Serialization;

namespace IEnumerableTest
{
    class Program
    {
        static void Main(string[] args)
        {
            DataContractSerializer ser =
                new DataContractSerializer(typeof(Test));

            using (FileStream writer = new FileStream("test.xml", FileMode.Create))
            {
                Test test = new Test();
                //NullReferenceException thrown by the next call
                //if YieldValues is flagged as [DataMember]
                ser.WriteObject(writer, test);
            }
        }
    }

    [DataContract(Name = "Test")]
    public class Test
    {
        List<int> values = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8 };

        //This property serializes without issue
        [DataMember]
        public IEnumerable<int> Values
        {
            get
            {
                return values;
            }
        }

        //Attempting to serialize this member results in a NullReferenceException
        [DataMember]
        public IEnumerable<int> YieldValues
        {
            get
            {
                foreach (int value in values)
                {
                    yield return value;
                }
            }
        }

        public Test()
        {

        }
    }
}

异常详情：

System.NullReferenceException was unhandled
  Message=Object reference not set to an instance of an object.
  Source=System.Runtime.Serialization
  StackTrace:
       at System.Runtime.Serialization.XmlObjectSerializerWriteContext.OnHandleIsReference(XmlWriterDelegator xmlWriter, DataContract contract, Object obj)
       at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithXsiType(XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle objectTypeHandle, Type objectType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle, Type declaredType)
       at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
       at WriteTestToXml(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.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.Runtime.Serialization.XmlObjectSerializer.WriteObject(Stream stream, Object graph)
       at IEnumerableTest.Program.Main(String[] args) in F:\Users\Caleb\Documents\Visual Studio 2010\Projects\IEnumerable\Program.cs:line 19
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

这是在 .NET 4.0 中，在 Visual Studio 2010 中运行

Answer 1

考虑以下顺序：

var file = File.Open("a.txt");
yield return "";
//#1
yield return new StreamReader(file).ReadToEnd();

想象一下这个序列被枚举到点 #1 并暂停。即使您能够序列化它，您将如何恢复它？ DataContractSerializer 无法神奇地重新打开您的文件。

没有安全的方法来恢复/反序列化 yield 支持的序列，因为这样的序列可以做任何事情。它可以打开一个消息框或格式化您的硬盘。

这就是为什么 C# 语言设计者没有在其迭代器类上公开任何可用于序列化或反序列化的功能。

仅使用反射手动序列化迭代器类上的字段将始终依赖于编译器实现细节。尚未准备好生产。