Json.NET different json structure, based on enum value

Question

I need to convert my class to JSON and I use Json.NET. But I can have different JSON structures, like:

{
    name: "Name",
    type: "simple1",
    value: 100
};

or

{
    name: "Name",
    type: {
        optional1: {
            setting1: "s1",
            setting2: "s2",
            ///etc.
    },
    value: 100
};

My C# code is:

public class Configuration
{
    [JsonProperty(PropertyName = "name")]
    public string Name{ get; set; }

    [JsonProperty(PropertyName = "type")]
    public MyEnumTypes Type { get; set; }

    public OptionalType TypeAdditionalData { get; set; }

    [JsonProperty(PropertyName = "value")]
    public int Value { get; set; }
    public bool ShouldSerializeType()
    {
        OptionalSettingsAttribute optionalSettingsAttr = this.Type.GetAttributeOfType<OptionalSettingsAttribute>();
        return optionalSettingsAttr == null;
    }

    public bool ShouldSerializeTypeAdditionalData()
    {
        OptionalSettingsAttribute optionalSettingsAttr = this.Type.GetAttributeOfType<OptionalSettingsAttribute>();
        return optionalSettingsAttr != null;
    }
}

public enum MyEnumTypes 
{
    [EnumMember(Value = "simple1")]
    Simple1,

    [EnumMember(Value = "simple2")]
    Simple2,

    [OptionalSettingsAttribute]
    [EnumMember(Value = "optional1")]
    Optional1,

    [EnumMember(Value = "optional2")]
    [OptionalSettingsAttribute]
    Optional2
}

My idea was when Configuration.Type - value hasn't attribute OptionalSettingsAttribute - to serialize it as type: "simple1". Otherwise - to use Configuration.Type - value as type's value key (type: { optional1: {} }) and value in Configuration.TypeAdditionalData as optional1 - value (like 2 simple JSON above).

I tried to create a custom Converter, like:

public class ConfigurationCustomConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return typeof(Configuration).IsAssignableFrom(objectType);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        return serializer.Deserialize<Configuration>(reader);
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        //my changes here

        serializer.Serialize(writer, value);
    }

But when I add [JsonConverter(typeof(ConfigurationCustomConverter))] attribute to Configuration class:

[JsonConverter(typeof(ConfigurationCustomConverter))]
public class Configuration

and called JsonConvert.SerializeObject(configurationObj); I received next error:

Self referencing loop detected with type 'Configuration'. Path ''.

Do you have any ideas how to change my code to serialize my class to 2 different JSON structures? Note: I won't use the same class to deserialize the JSON.

Thank you!

Answer 1

The reason you are getting the Self referencing loop detected exception is that the WriteJson method of your converter is calling itself recursively. When you apply a converter to a type using [JsonConverter(typeof(ConfigurationCustomConverter))], the WriteJson() method will unconditionally replace Json.NET's default implementation. Thus your inner call:

public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
    //my changes here
    serializer.Serialize(writer, value);
}

would cause a stack overflow. Json.NET notices this and instead throws the exception you see. For more details, see JSON.Net throws StackOverflowException when using [JsonConvert()]. Setting ReferenceLoopHandling.Ignore simply causes the infinite recursion to be skipped, leaving your object empty.

You have a few options to solve this problem:

1. You could manually write all property names and values other than Type and TypeAdditionalData then write out the custom "type" property last. For instance:

   [JsonConverter(typeof(ConfigurationConverter))]
   public class Configuration
   {
       [JsonProperty(PropertyName = "name")]
       public string Name { get; set; }
   
       public MyEnumTypes Type { get; set; }
   
       public OptionalType TypeAdditionalData { get; set; }
   
       [JsonProperty(PropertyName = "value")]
       public int Value { get; set; }
   }
   
   class ConfigurationConverter : JsonConverter
   {
       const string typeName = "type";
   
       public override bool CanConvert(Type objectType)
       {
           return typeof(Configuration).IsAssignableFrom(objectType);
       }
   
       public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
       {
           if (reader.TokenType == JsonToken.Null)
               return null;
           var config = (existingValue as Configuration ?? (Configuration)serializer.ContractResolver.ResolveContract(objectType).DefaultCreator());
   
           // Populate the regular property values.
           var obj = JObject.Load(reader);
           var type = obj.RemoveProperty(typeName);
           using (var subReader = obj.CreateReader())
               serializer.Populate(subReader, config);
   
           // Populate Type and OptionalType
           if (type is JValue) // Primitive value
           {
               config.Type = type.ToObject<MyEnumTypes>(serializer);
           }
           else
           {
               var dictionary = type.ToObject<Dictionary<MyEnumTypes, OptionalType>>(serializer);
               if (dictionary.Count > 0)
               {
                   config.Type = dictionary.Keys.First();
                   config.TypeAdditionalData = dictionary.Values.First();
               }
           }
   
           return config;
       }
   
       public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
       {
           var config = (Configuration)value;
           var contract = (JsonObjectContract)serializer.ContractResolver.ResolveContract(config.GetType());
           writer.WriteStartObject();
           foreach (var property in contract.Properties
               .Where(p => p.Writable && (p.ShouldSerialize == null || p.ShouldSerialize(config)) && !p.Ignored))
           {
               if (property.UnderlyingName == "Type" || property.UnderlyingName == "TypeAdditionalData")
                   continue;
               var propertyValue = property.ValueProvider.GetValue(config);
               if (propertyValue == null && serializer.NullValueHandling == NullValueHandling.Ignore)
                   continue;
               writer.WritePropertyName(property.PropertyName);
               serializer.Serialize(writer, propertyValue);
           }
           writer.WritePropertyName(typeName);
           if (config.Type.GetCustomAttributeOfEnum<OptionalSettingsAttribute>() == null)
           {
               serializer.Serialize(writer, config.Type);
           }
           else
           {
               var dictionary = new Dictionary<MyEnumTypes, OptionalType>
               {
                   { config.Type, config.TypeAdditionalData },
               };
               serializer.Serialize(writer, dictionary);
           }
           writer.WriteEndObject();
       }
   }
   
   public class OptionalType
   {
       public string setting1 { get; set; }
   }
   
   public class OptionalSettingsAttribute : System.Attribute
   {
       public OptionalSettingsAttribute()
       {
       }
   }
   
   [JsonConverter(typeof(StringEnumConverter))]
   public enum MyEnumTypes
   {
       [EnumMember(Value = "simple1")]
       Simple1,
   
       [EnumMember(Value = "simple2")]
       Simple2,
   
       [OptionalSettingsAttribute]
       [EnumMember(Value = "optional1")]
       Optional1,
   
       [EnumMember(Value = "optional2")]
       [OptionalSettingsAttribute]
       Optional2
   }
   
   public static class EnumExtensions
   {
       public static TAttribute GetCustomAttributeOfEnum<TAttribute>(this Enum value)
           where TAttribute : System.Attribute
       {
           var type = value.GetType();
           var memInfo = type.GetMember(value.ToString());
           return memInfo[0].GetCustomAttribute<TAttribute>();
       }
   }
   
   public static class JsonExtensions
   {
       public static JToken RemoveProperty(this JObject obj, string name)
       {
           if (obj == null)
               return null;
           var property = obj.Property(name);
           if (property == null)
               return null;
           var value = property.Value;
           property.Remove();
           property.Value = null;
           return value;
       }
   }
   

   Notice I added [JsonConverter(typeof(StringEnumConverter))] to your enum. This ensures the type is always written as a string.

   Sample fiddle.

2. You could disable recursive calls to the converter via the technique shown in JSON.Net throws StackOverflowException when using [JsonConvert()], generate a default serialization, modify it as required, and write it out.

3. You could avoid the use of a converter entirely by marking Type and TypeAdditionalData as [JsonIgnore] and introducing an additional private property to serialize and deserialize "type":

   public class Configuration
   {
       [JsonProperty(PropertyName = "name")]
       public string Name { get; set; }
   
       [JsonIgnore]
       public MyEnumTypes Type { get; set; }
   
       [JsonIgnore]
       public OptionalType TypeAdditionalData { get; set; }
   
       [JsonProperty("type")]
       JToken SerializedType
       {
           get
           {
               if (Type.GetCustomAttributeOfEnum<OptionalSettingsAttribute>() == null)
               {
                   return JToken.FromObject(Type);
               }
               else
               {
                   var dictionary = new Dictionary<MyEnumTypes, OptionalType>
                   {
                       { Type, TypeAdditionalData },
                   };
                   return JToken.FromObject(dictionary);
               }
           }
           set
           {
               if (value == null || value.Type == JTokenType.Null)
               {
                   TypeAdditionalData = null;
                   Type = default(MyEnumTypes);
               }
               else if (value is JValue)
               {
                   Type = value.ToObject<MyEnumTypes>();
               }
               else
               {
                   var dictionary = value.ToObject<Dictionary<MyEnumTypes, OptionalType>>();
                   if (dictionary.Count > 0)
                   {
                       Type = dictionary.Keys.First();
                       TypeAdditionalData = dictionary.Values.First();
                   }
               }
           }
       }
   
       [JsonProperty(PropertyName = "value")]
       public int Value { get; set; }
   }