如何将json对象反序列化为特定的子类？

我有一个Cabin类，其中包含Row对象列表。 我想序列化这样的对象，但是当反序列化时，我希望Row对象是RowRule对象，它inheritance自Row对象。 下面是我一直在尝试的一些示例代码。

class Program { static void Main(string[] args) { var cabin = new Cabin(); var row = new Row(); row.Status = "Success"; cabin.Rows = new List() { row, row }; JsonSerializerSettings settings = new JsonSerializerSettings() { TypeNameHandling = TypeNameHandling.Auto }; string json = JsonConvert.SerializeObject(cabin, Formatting.Indented, settings); Console.WriteLine(json); Cabin obj = JsonConvert.DeserializeObject(json, new JsonSerializerSettings() {TypeNameHandling = TypeNameHandling.Auto}); Console.WriteLine(obj); Debug.Assert(obj.Rows.First().GetType().Name == "RowRule"); } } class Cabin { public IList Rows { get; set; } } class Row { public string Status { get; set; } } class RowRule : Row { } 

Answer 1

简单的答案是使用CustomCreationConverter并从Create()返回RowRule ：

 class RowToRoleRuleConverter : CustomCreationConverter { public override Row Create(Type objectType) { if (objectType.IsAssignableFrom(typeof(RowRule))) return Activator.CreateInstance(); return (Row)Activator.CreateInstance(objectType); } } 

但是，您使用的是TypeNameHandling.Auto ，这意味着您的JSON中可能存在多态"$type"属性。 不幸的是， CustomCreationConverter忽略了这些属性。 因此，有必要做一些额外的工作并创建DowncastingConverter ：

 public class DowncastingConverter : PolymorphicCreationConverter where TDerived : TBase { protected override TBase Create(Type objectType, Type polymorphicType, object existingValue, IContractResolver contractResolver, JObject obj) { Type createType = objectType; if (createType.IsAssignableFrom(polymorphicType)) createType = polymorphicType; if (createType.IsAssignableFrom(typeof(TDerived))) createType = typeof(TDerived); if (existingValue != null && createType.IsAssignableFrom(existingValue.GetType())) return (TBase)existingValue; var contract = contractResolver.ResolveContract(createType); return (TBase)contract.DefaultCreator(); } } public abstract class PolymorphicCreationConverter : JsonConverter { public override bool CanConvert(Type objectType) { return typeof(T).IsAssignableFrom(objectType); } public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) { throw new NotSupportedException("CustomCreationConverter should only be used while deserializing."); } public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) { if (reader.TokenType == JsonToken.Null) return null; var obj = JObject.Load(reader); Type polymorphicType = null; var polymorphicTypeString = (string)obj["$type"]; if (polymorphicTypeString != null) { if (serializer.TypeNameHandling != TypeNameHandling.None) { string typeName, assemblyName; ReflectionUtils.SplitFullyQualifiedTypeName(polymorphicTypeString, out typeName, out assemblyName); polymorphicType = serializer.Binder.BindToType(assemblyName, typeName); } obj.Remove("$type"); } var value = Create(objectType, polymorphicType, existingValue, serializer.ContractResolver, obj); if (value == null) throw new JsonSerializationException("No object created."); using (var subReader = obj.CreateReader()) serializer.Populate(subReader, value); return value; } protected abstract T Create(Type objectType, Type polymorphicType, object existingValue, IContractResolver iContractResolver, JObject obj); public override bool CanWrite { get { return false; } } } internal static class ReflectionUtils { // Utilities taken from https://github.com/JamesNK/Newtonsoft.Json/blob/master/Src/Newtonsoft.Json/Utilities/ReflectionUtils.cs // I couldn't find a way to access these directly. public static void SplitFullyQualifiedTypeName(string fullyQualifiedTypeName, out string typeName, out string assemblyName) { int? assemblyDelimiterIndex = GetAssemblyDelimiterIndex(fullyQualifiedTypeName); if (assemblyDelimiterIndex != null) { typeName = fullyQualifiedTypeName.Substring(0, assemblyDelimiterIndex.GetValueOrDefault()).Trim(); assemblyName = fullyQualifiedTypeName.Substring(assemblyDelimiterIndex.GetValueOrDefault() + 1, fullyQualifiedTypeName.Length - assemblyDelimiterIndex.GetValueOrDefault() - 1).Trim(); } else { typeName = fullyQualifiedTypeName; assemblyName = null; } } private static int? GetAssemblyDelimiterIndex(string fullyQualifiedTypeName) { int scope = 0; for (int i = 0; i < fullyQualifiedTypeName.Length; i++) { char current = fullyQualifiedTypeName[i]; switch (current) { case '[': scope++; break; case ']': scope--; break; case ',': if (scope == 0) { return i; } break; } } return null; } } 

然后使用它像：

 JsonSerializerSettings readSettings = new JsonSerializerSettings() { TypeNameHandling = TypeNameHandling.Auto, Converters = new[] { new DowncastingConverter() }, }; Cabin obj = JsonConvert.DeserializeObject(json, readSettings); 

原型小提琴 。

最后，在使用TypeNameHandling ，请注意Newtonsoft文档中的这一注意事项：

当您的应用程序从外部源反序列化JSON时，应谨慎使用TypeNameHandling。 使用非None以外的值进行反序列化时，应使用自定义SerializationBindervalidation传入类型。

有关可能需要这样做的讨论，请参阅Newtonsoft Json中的TypeNameHandling警告 。

Answer 2

您的示例代码的问题是，您正在创建Row对象并尝试获取RowRule ，这是不可能的。

可能你想这样做：

  var cabin = new Cabin(); var row = new RowRule(); // create derived object row.Status = "Success"; cabin.Rows = new List() { row, row };