c# - Deserialize specific enum into system.enum in Json.Net

Question

I have a fairly generic 'rule' class that I am using to drive the behavior of an analysis engine I'm writing:

public class Rule
{
    /// <summary>
    /// The general rule type.
    /// </summary>
    public RuleType RuleType { get; set; }

    /// <summary>
    /// The human-readable description of the rule.
    /// </summary>
    public string RuleDescription { get; set; }

    /// <summary>
    /// The integer magnitude of the rule, if applicable.
    /// </summary>
    public int? RuleInt { get; set; }

    /// <summary>
    /// The boolean sign associated with the rule, if applicable.
    /// </summary>
    public bool? RuleBool { get; set; }

    /// <summary>
    /// The enum flag associated with the rule, if applicable.  CAN be null.
    /// </summary>
    public System.Enum RuleFlagEnum { get; set; }

    /// <summary>
    /// A dumping ground for any other random crap I've failed to account for at this point in time.
    /// </summary>
    public object RuleObject { get; set; }
}

RuleType is a specific enum, like so:

public enum RuleType
{
    Invalid,
    ModifyDifficulty,
    StrengthChange,
    ColorChange,
    SignChange
}

Using Json.NET, that both serializes and deserializes just fine.

RuleEnum, however, is giving me problems. Whether using the default enum serialization or the string enum serialization, the specific type of enum is not provided. As such, during deserialization, I am left with System.Enum and a string value, which is wholly unhelpful.

This is an example of the serialization, to show what I'm talking about:

{
   "RuleType": "SignChange",
   "RuleDescription": "Strength 1 Inversion Gate",
   "RuleInt": 1,
   "RuleFlagEnum": "Negative"
}

RuleFlagEnum, in this case, is referring to the enum:

public enum SignChange
{
    Zero,
    Positive,
    Negative
}

I have tried using all of the TypeNameHandling options inside Json.NET. They only put type hinting on the objects, which doesn't help with RuleFlagEnum since it is technically a primitive.

I would really, really like to keep the enum at System.Enum so we can load any arbitrary enum in for later interpretation by the rule type, so the entire thing is more expandable. Is this possible?

Answer 1

The difficulty here is that System.Enum is an abstract class, so it is impossible to deserialize a value of unknown concrete type as such a type. Rather, one needs to have the specific type information in the JSON somewhere, however Json.NET will serialize an enum as a string or an integer (depending upon whether a StringEnumConverter is applied) -- but not an as an object, thus leaving no opportunity for a polymorphic "$type" property to be added.

The solution is, when serializing, to serialize a generic wrapper class that can convey the concrete type information:

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 serialize the wrapper when serializing your class:

    /// <summary>
    /// The enum flag associated with the rule, if applicable.  CAN be null.
    /// </summary>
    [JsonIgnore]
    public System.Enum RuleFlagEnum { get; set; }

    [JsonProperty("RuleFlagEnum", TypeNameHandling = TypeNameHandling.All)]
    TypeWrapper RuleFlagEnumValue
    {
        get
        {
            return RuleFlagEnum == null ? null : TypeWrapper.CreateWrapper(RuleFlagEnum);
        }
        set
        {
            if (value == null || value.ObjectValue == null)
                RuleFlagEnum = null;
            else
                RuleFlagEnum = (Enum)value.ObjectValue;
        }
    }

This produces JSON like the following:

{
  "RuleType": "ModifyDifficulty",
  "RuleFlagEnum": {
    "$type": "Question31351262.TypeWrapper`1[[Question31351262.MyEnum, MyApp]], MyApp",
    "Value": "Two, Three"
  },
}