Question

如何转换具有递归类型的对象（不是对象，只是类型）？尝试使用父/子关系序列化对象时，以下代码失败。

错误是对 ser.WriteObject（stream1，parent）的最后一次调用; 会抛出异常：

System.Runtime.Serialization.SerializationException was unhandled
  HResult=-2146233076
  Message=Type 'TestJson.Child' with data contract name 'Child:http://schemas.datacontract.org/2004/07/TestJson' is not expected. Consider using a DataContractResolver or add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.
  Source=System.Runtime.Serialization
  StackTrace:
       at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeAndVerifyType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, Boolean verifyKnownType, RuntimeTypeHandle declaredTypeHandle, Type declaredType)
       at System.Runtime.Serialization.Json.XmlObjectSerializerWriteContextComplexJson.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 System.Runtime.Serialization.XmlObjectSerializerWriteContextComplex.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
       at WriteArrayOfPersonToJson(XmlWriterDelegator , Object , XmlObjectSerializerWriteContextComplexJson , CollectionDataContract )
       at System.Runtime.Serialization.Json.JsonCollectionDataContract.WriteJsonValueCore(XmlWriterDelegator jsonWriter, Object obj, XmlObjectSerializerWriteContextComplexJson context, RuntimeTypeHandle declaredTypeHandle)
       at System.Runtime.Serialization.Json.XmlObjectSerializerWriteContextComplexJson.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 System.Runtime.Serialization.XmlObjectSerializerWriteContextComplex.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
       at WriteAdultToJson(XmlWriterDelegator , Object , XmlObjectSerializerWriteContextComplexJson , ClassDataContract , XmlDictionaryString[] )
       at System.Runtime.Serialization.Json.JsonClassDataContract.WriteJsonValueCore(XmlWriterDelegator jsonWriter, Object obj, XmlObjectSerializerWriteContextComplexJson context, RuntimeTypeHandle declaredTypeHandle)
       at System.Runtime.Serialization.Json.XmlObjectSerializerWriteContextComplexJson.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.Json.DataContractJsonSerializer.InternalWriteObjectContent(XmlWriterDelegator writer, Object graph)
       at System.Runtime.Serialization.Json.DataContractJsonSerializer.InternalWriteObject(XmlWriterDelegator writer, Object graph)
       at System.Runtime.Serialization.XmlObjectSerializer.InternalWriteObject(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
       at System.Runtime.Serialization.XmlObjectSerializer.WriteObjectHandleExceptions(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
       at System.Runtime.Serialization.Json.DataContractJsonSerializer.WriteObject(XmlDictionaryWriter writer, Object graph)
       at System.Runtime.Serialization.Json.DataContractJsonSerializer.WriteObject(Stream stream, Object graph)
       at TestJson.Program.Main(String[] args) in c:\src\TestJson\Program.cs:line 44
       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.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException:

完整的代码是：

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

namespace TestJson
{
    class Program
    {
        static void Main(string[] args)
        {
            Adult parent = new Adult {name = "John", age = 42};

            MemoryStream stream1 = new MemoryStream();
            DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(Adult));
            ser.WriteObject(stream1, parent);

            stream1.Position = 0;
            StreamReader sr = new StreamReader(stream1);
            Console.Write("JSON form of Adult with no children: ");
            Console.WriteLine(sr.ReadToEnd());


            Child child = new Child { name = "Jane", age = 4, fingers=10 };

            stream1 = new MemoryStream();
            ser = new DataContractJsonSerializer(typeof(Child));
            ser.WriteObject(stream1, child);

            stream1.Position = 0;
            sr = new StreamReader(stream1);
            Console.Write("JSON form of Child with no parent: ");
            Console.WriteLine(sr.ReadToEnd());


            // now connect the two
            parent.children.Add(child);

            stream1 = new MemoryStream();
            ser = new DataContractJsonSerializer(typeof(Adult));
            ser.WriteObject(stream1, parent);

            stream1.Position = 0;
            sr = new StreamReader(stream1);
            Console.Write("JSON form of Adult with 1 child: ");
            Console.WriteLine(sr.ReadToEnd());
        }
    }

    [DataContract]
    class Person
    {
        [DataMember]
        internal string name;

        [DataMember]
        internal int age;
    }

    [DataContract]
    class Adult : Person
    {
        [DataMember] 
        internal List<Person> children = new List<Person>();
    }

    [DataContract]
    class Child : Person
    {
        [DataMember]
        internal int fingers;
    }
}

Answer 1

只需按照异常消息中提供的建议操作，并将KnownType属性添加到Person：

[DataContract]
[KnownType(typeof(Parent))]
[KnownType(typeof(Child))]
class Person
{
    [DataMember]
    internal string name;

    [DataMember]
    internal int age;
}

您需要通过基类的KnownType属性主动使序列化引擎知道派生类型。
at 与递归对象无关。

如何将递归类型转换为JSON

1 个答案: