How to deserialize json objects into specific subclasses?

Question

I have a Cabin class that contains a list of Row objects. I\'d like to serialize the objects like this, but when deserializing I\'d like the Row objects to be RowRule objec

Answer 1

The problem with your sample code is, you're creating object of Row and trying to get RowRule which is not possible.

May be you wanted to do it like this:

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

Answer 2

The simple answer is to use a CustomCreationConverter<Row> and return a RowRule from Create():

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

However, you are using TypeNameHandling.Auto which implies that there may be polymorphic "$type" properties in your JSON. Unfortunately, CustomCreationConverter<T> ignores these properties. Thus it will be necessary to do some additional work and create DowncastingConverter<TBase, TDerived>:

public class DowncastingConverter<TBase, TDerived> : PolymorphicCreationConverter<TBase> 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<T> : 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;
    }
}

Then use it like:

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

Prototype fiddle.

Finally, when using TypeNameHandling, do take note of this caution from the Newtonsoft docs:

TypeNameHandling should be used with caution when your application deserializes JSON from an external source. Incoming types should be validated with a custom SerializationBinder when deserializing with a value other than None.

For a discussion of why this may be necessary, see TypeNameHandling caution in Newtonsoft Json.