serialization - Deserialize Dictionary<string, object> with enum values in C#

Question

I am trying to serialize/deserialize a Dictionary<string, object> in C#. Object can be anything that is serializable.

Json.NET almost works, but if a value within the dictionary is an enum, the deserialization is not correct, as it gets deserialized as a long. TypeNameHandling.All does not make any difference.

Is there any other fast solution to serialization library. The result does not have to be JSON, but must be text.

I also have no influence on the data that is passed to the dictionary. I just have to serialize and deserialize anything that comes into my way.

EDIT: StringEnumConverter does not help. The data gets converted back to Dictionary<string, object>, so the deserializer does not know that the serialized value is an enum. It treats it like an object, with StringEnumConverter it remains a string when deserialized; it gets deserialized as a long without the converter. JSON.NET does not preserve the enum.

The solution i want to provide is an implementation of an existing interface that gets injected into an existing solution that i cannot change.

EDIT2: Here is an example of what i am trying to do

public enum Foo { A, B, C }
public enum Bar { A, B, C }
public class Misc { public Foo Foo { get; set; } }


var dict = new Dictionary<string, object>();
dict.Add("a", Foo.A);
dict.Add("b", Bar.B);
dict.Add("c", new Misc());

// serialize dict to a string s
// deserialize s to a Dictionary<string, object> dict2

Assert.AreEqual(Foo.A, dict2["a"]);
Assert.AreEqual(Bar.B, dict2["b"]);

Important: i cannot control dict; it is actually a custom type that is derived from Dictionary<string, object>: I just have to make sure that all keys and values deserialized are from the same type when deserialized, so that no casting is needed. And again, i do not have to use JSON; maybe there is some other serializer that can handle the job!?

Answer 1

Presumably you are already serializing your dictionary with TypeNameHandling.All, which should correctly serialize and deserialize the new Misc() value by emitting a "$type" object property along with the object itself. Unfortunately, for types such as enums (and others such as as int and long), this doesn't work because these are serialized as JSON primitives, with no opportunity to include a "$type" property.

The solution is, when serializing a dictionary with object values, to serialize object wrappers for primitive values that can encapsulate the type information, along the lines of this answer. Since you cannot modify any of your incoming objects and need to "inject" the proper wrappers, you can do this with a custom contract resolver that applies an appropriate item converter to the dictionary values:

public class UntypedToTypedValueContractResolver : DefaultContractResolver
{
    // As of 7.0.1, Json.NET suggests using a static instance for "stateless" contract resolvers, for performance reasons.
    // http://www.newtonsoft.com/json/help/html/ContractResolver.htm
    // http://www.newtonsoft.com/json/help/html/M_Newtonsoft_Json_Serialization_DefaultContractResolver__ctor_1.htm
    // "Use the parameterless constructor and cache instances of the contract resolver within your application for optimal performance."
    // See also https://stackoverflow.com/questions/33557737/does-json-net-cache-types-serialization-information
    static UntypedToTypedValueContractResolver instance;

    // Explicit static constructor to tell C# compiler not to mark type as beforefieldinit
    static UntypedToTypedValueContractResolver() { instance = new UntypedToTypedValueContractResolver(); }

    public static UntypedToTypedValueContractResolver Instance { get { return instance; } }

    protected override JsonDictionaryContract CreateDictionaryContract(Type objectType)
    {
        var contract = base.CreateDictionaryContract(objectType);

        if (contract.DictionaryValueType == typeof(object) && contract.ItemConverter == null)
        {
            contract.ItemConverter = new UntypedToTypedValueConverter();
        }

        return contract;
    }
}

class UntypedToTypedValueConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        throw new NotImplementedException("This converter should only be applied directly via ItemConverterType, not added to JsonSerializer.Converters");
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.Null)
            return null;
        var value = serializer.Deserialize(reader, objectType);
        if (value is TypeWrapper)
        {
            return ((TypeWrapper)value).ObjectValue;
        }
        return value;
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        if (serializer.TypeNameHandling == TypeNameHandling.None)
        {
            Debug.WriteLine("ObjectItemConverter used when serializer.TypeNameHandling == TypeNameHandling.None");
            serializer.Serialize(writer, value);
        }
        // Handle a couple of simple primitive cases where a type wrapper is not needed
        else if (value is string)
        {
            writer.WriteValue((string)value);
        }
        else if (value is bool)
        {
            writer.WriteValue((bool)value);
        }
        else
        {
            var contract = serializer.ContractResolver.ResolveContract(value.GetType());
            if (contract is JsonPrimitiveContract)
            {
                var wrapper = TypeWrapper.CreateWrapper(value);
                serializer.Serialize(writer, wrapper, typeof(object));
            }
            else
            {
                serializer.Serialize(writer, value);
            }
        }
    }
}

public abstract class TypeWrapper
{
    protected TypeWrapper() { }

    [JsonIgnore]
    public abstract object ObjectValue { get; }

    public static TypeWrapper CreateWrapper<T>(T value)
    {
        if (value == null)
            return new TypeWrapper<T>();
        var type = value.GetType();
        if (type == typeof(T))
            return new TypeWrapper<T>(value);
        // Return actual type of subclass
        return (TypeWrapper)Activator.CreateInstance(typeof(TypeWrapper<>).MakeGenericType(type), value);
    }
}

public sealed class TypeWrapper<T> : TypeWrapper
{
    public TypeWrapper() : base() { }

    public TypeWrapper(T value)
        : base()
    {
        this.Value = value;
    }

    public override object ObjectValue { get { return Value; } }

    public T Value { get; set; }
}

Then use it like:

var settings = new JsonSerializerSettings
{
    TypeNameHandling = TypeNameHandling.All,
    ContractResolver = UntypedToTypedValueContractResolver.Instance,
    Converters = new [] { new StringEnumConverter() }, // If you prefer
};

var json = JsonConvert.SerializeObject(dict, Formatting.Indented, settings);

var dict2 = JsonConvert.DeserializeObject<Dictionary<string, object>>(json, settings);

Sample 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.