Deserializing polymorphic json classes without type information using json.net

Question

This Imgur api call returns a list containing both Gallery Image and Gallery Album classes represented in JSON.

I can't see how to deserialize these automatically using Json.NET given that there is no $type property telling the deserializer which class is meant to be represented. There is a property called "IsAlbum" that can be used to differentiate between the two.

This question appears to show one method but it looks like a bit of a hack.

How do I go about deserializing these classes? (using C#, Json.NET).

Sample Data:

Gallery Image

{
    "id": "OUHDm",
    "title": "My most recent drawing. Spent over 100 hours.",
        ...
    "is_album": false
}

Gallery Album

{
    "id": "lDRB2",
    "title": "Imgur Office",
    ...
    "is_album": true,
    "images_count": 3,
    "images": [
        {
            "id": "24nLu",
            ...
            "link": "http://i.imgur.com/24nLu.jpg"
        },
        {
            "id": "Ziz25",
            ...
            "link": "http://i.imgur.com/Ziz25.jpg"
        },
        {
            "id": "9tzW6",
            ...
            "link": "http://i.imgur.com/9tzW6.jpg"
        }
    ]
}
}

Answer 1

You can do this fairly easily by creating a custom JsonConverter to handle the object instantiation. Assuming you have your classes defined something like this:

public abstract class GalleryItem
{
    public string id { get; set; }
    public string title { get; set; }
    public string link { get; set; }
    public bool is_album { get; set; }
}

public class GalleryImage : GalleryItem
{
    // ...
}

public class GalleryAlbum : GalleryItem
{
    public int images_count { get; set; }
    public List<GalleryImage> images { get; set; }
}

You would create the converter like this:

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

    public override object ReadJson(JsonReader reader, 
        Type objectType, object existingValue, JsonSerializer serializer)
    {
        JObject jo = JObject.Load(reader);

        // Using a nullable bool here in case "is_album" is not present on an item
        bool? isAlbum = (bool?)jo["is_album"];

        GalleryItem item;
        if (isAlbum.GetValueOrDefault())
        {
            item = new GalleryAlbum();
        }
        else
        {
            item = new GalleryImage();
        }

        serializer.Populate(jo.CreateReader(), item);

        return item;
    }

    public override bool CanWrite
    {
        get { return false; }
    }

    public override void WriteJson(JsonWriter writer, 
        object value, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

Here's an example program showing the converter in action:

class Program
{
    static void Main(string[] args)
    {
        string json = @"
        [
            {
                ""id"": ""OUHDm"",
                ""title"": ""My most recent drawing. Spent over 100 hours."",
                ""link"": ""http://i.imgur.com/OUHDm.jpg"",
                ""is_album"": false
            },
            {
                ""id"": ""lDRB2"",
                ""title"": ""Imgur Office"",
                ""link"": ""http://alanbox.imgur.com/a/lDRB2"",
                ""is_album"": true,
                ""images_count"": 3,
                ""images"": [
                    {
                        ""id"": ""24nLu"",
                        ""link"": ""http://i.imgur.com/24nLu.jpg""
                    },
                    {
                        ""id"": ""Ziz25"",
                        ""link"": ""http://i.imgur.com/Ziz25.jpg""
                    },
                    {
                        ""id"": ""9tzW6"",
                        ""link"": ""http://i.imgur.com/9tzW6.jpg""
                    }
                ]
            }
        ]";

        List<GalleryItem> items = 
            JsonConvert.DeserializeObject<List<GalleryItem>>(json, 
                new GalleryItemConverter());

        foreach (GalleryItem item in items)
        {
            Console.WriteLine("id: " + item.id);
            Console.WriteLine("title: " + item.title);
            Console.WriteLine("link: " + item.link);
            if (item.is_album)
            {
                GalleryAlbum album = (GalleryAlbum)item;
                Console.WriteLine("album images (" + album.images_count + "):");
                foreach (GalleryImage image in album.images)
                {
                    Console.WriteLine("    id: " + image.id);
                    Console.WriteLine("    link: " + image.link);
                }
            }
            Console.WriteLine();
        }
    }
}

And here is the output of the above program:

id: OUHDm
title: My most recent drawing. Spent over 100 hours.
link: http://i.imgur.com/OUHDm.jpg

id: lDRB2
title: Imgur Office
link: http://alanbox.imgur.com/a/lDRB2
album images (3):
    id: 24nLu
    link: http://i.imgur.com/24nLu.jpg
    id: Ziz25
    link: http://i.imgur.com/Ziz25.jpg
    id: 9tzW6
    link: http://i.imgur.com/9tzW6.jpg

Fiddle: https://dotnetfiddle.net/1kplME

Answer 2

Simply with JsonSubTypes attributes that work with Json.NET

    [JsonConverter(typeof(JsonSubtypes), "is_album")]
    [JsonSubtypes.KnownSubType(typeof(GalleryAlbum), true)]
    [JsonSubtypes.KnownSubType(typeof(GalleryImage), false)]
    public abstract class GalleryItem
    {
        public string id { get; set; }
        public string title { get; set; }
        public string link { get; set; }
        public bool is_album { get; set; }
    }

    public class GalleryImage : GalleryItem
    {
        // ...
    }

    public class GalleryAlbum : GalleryItem
    {
        public int images_count { get; set; }
        public List<GalleryImage> images { get; set; }
    }

Answer 3

Advanced to Brian Rogers answer. And about "use Serializer.Populate() instead of item.ToObject()". If derived types has contstructors or some of their has own customconverter you must use general way for deserialize JSON. So you must leave work for instantiate new object to NewtonJson. This way you can achieve it in you CustomJsonConverter:

public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
    ..... YOU Code For Determine Real Type of Json Record .......

    // 1. Correct ContractResolver for you derived type
    var contract = serializer.ContractResolver.ResolveContract(DeterminedType);
    if (converter != null && !typeDeserializer.Type.IsAbstract && converter.GetType() == GetType())
    {
        contract.Converter = null; // Clean Wrong Converter grabbed by DefaultContractResolver from you base class for derived class
    }

    // Deserialize in general way           
    var jTokenReader = new JTokenReader(jObject);
    var result = serializer.Deserialize(jTokenReader, DeterminedType);

    return (result);
}

This work if you have recursion of objects.

Answer 4

I'm only posting this to clear up some of the confusion. If you are working with a predefined format and need to deserialize it, this is what I found worked best and demonstrates the mechanics so that others can tweak it as needed.

public class BaseClassConverter : JsonConverter
    {
        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            var j = JObject.Load(reader);
            var retval = BaseClass.From(j, serializer);
            return retval;
        }

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

        public override bool CanConvert(Type objectType)
        {
            // important - do not cause subclasses to go through this converter
            return objectType == typeof(BaseClass);
        }
    }

    // important to not use attribute otherwise you'll infinite loop
    public abstract class BaseClass
    {
        internal static Type[] Types = new Type[] {
            typeof(Subclass1),
            typeof(Subclass2),
            typeof(Subclass3)
        };

        internal static Dictionary<string, Type> TypesByName = Types.ToDictionary(t => t.Name.Split('.').Last());

        // type property based off of class name
        [JsonProperty(PropertyName = "type", Required = Required.Always)]
        public string JsonObjectType { get { return this.GetType().Name.Split('.').Last(); } set { } }

        // convenience method to deserialize a JObject
        public static new BaseClass From(JObject obj, JsonSerializer serializer)
        {
            // this is our object type property
            var str = (string)obj["type"];

            // we map using a dictionary, but you can do whatever you want
            var type = TypesByName[str];

            // important to pass serializer (and its settings) along
            return obj.ToObject(type, serializer) as BaseClass;
        }


        // convenience method for deserialization
        public static BaseClass Deserialize(JsonReader reader)
        {
            JsonSerializer ser = new JsonSerializer();
            // important to add converter here
            ser.Converters.Add(new BaseClassConverter());

            return ser.Deserialize<BaseClass>(reader);
        }
    }

Answer 5

Following implementation should let you de-serialize without changing the way you have designed your classes and by using a field other than $type to decide what to de-serialize it into.

public class GalleryImageConverter : JsonConverter
{   
    public override bool CanConvert(Type objectType)
    {
        return (objectType == typeof(GalleryImage) || objectType == typeof(GalleryAlbum));
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        try
        {
            if (!CanConvert(objectType))
                throw new InvalidDataException("Invalid type of object");
            JObject jo = JObject.Load(reader);
            // following is to avoid use of magic strings
            var isAlbumPropertyName = ((MemberExpression)((Expression<Func<GalleryImage, bool>>)(s => s.is_album)).Body).Member.Name;
            JToken jt;
            if (!jo.TryGetValue(isAlbumPropertyName, StringComparison.InvariantCultureIgnoreCase, out jt))
            {
                return jo.ToObject<GalleryImage>();
            }
            var propValue = jt.Value<bool>();
            if(propValue) {
                resultType = typeof(GalleryAlbum);
            }
            else{
                resultType = typeof(GalleryImage);
            }
            var resultObject = Convert.ChangeType(Activator.CreateInstance(resultType), resultType);
            var objectProperties=resultType.GetProperties();
            foreach (var objectProperty in objectProperties)
            {
                var propType = objectProperty.PropertyType;
                var propName = objectProperty.Name;
                var token = jo.GetValue(propName, StringComparison.InvariantCultureIgnoreCase);
                if (token != null)
                {
                    objectProperty.SetValue(resultObject,token.ToObject(propType)?? objectProperty.GetValue(resultObject));
                }
            }
            return resultObject;
        }
        catch (Exception ex)
        {
            throw;
        }
    }

    public override bool CanWrite
    {
        get { return false; }
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

Answer 6

@ИгорьОрлов's answer works for when you have types that can only be instantiated directly by JSON.net (due to [JsonConstructor] and/or use of [JsonProperty] directly on constructor parameters. However overwriting contract.Converter = null does not work when JSON.net has already cached the converter to use.

_{(This wouldn't be an issue if JSON.NET used immutable types to indicate when data and configuration is no-longer mutable, le sigh)}

In my case, I did this:

1. Implemented a custom JsonConverter<T> (where T is my DTO's base class).
2. Defined a DefaultContractResolver subclass that overrides ResolveContractConverter to return my custom JsonConverter for only the base class.

In detail, and by example:

Supposing I have these immutable DTOs that represent a remote file-system (so there' DirectoryDto and FileDto which both inherit FileSystemDto, just like how DirectoryInfo and FileInfo derive from System.IO.FileSystemInfo):

public enum DtoKind
{
    None = 0,
    File,
    Directory
}

public abstract class FileSystemDto
{
    protected FileSystemDto( String name, DtoKind kind )
    {
        this.Name = name ?? throw new ArgumentNullException(nameof(name));
        this.Kind = kind;
    }

    [JsonProperty( "name" )]
    public String Name { get; }

    [JsonProperty( "kind" )]
    public String Kind { get; }
}

public class FileDto : FileSystemDto
{
    [JsonConstructor]
    public FileDto(
        [JsonProperty("name"  )] String  name,
        [JsonProperty("length")] Int64   length,
        [JsonProperty("kind")  ] DtoKind kind
    )
        : base( name: name, kind: kind )
    {
        if( kind != DtoKind.File ) throw new InvalidOperationException( "blargh" );
        this.Length = length;
    }

    [JsonProperty( "length" )]
    public Int64 Length { get; }
}

public class DirectoryDto : FileSystemDto
{
    [JsonConstructor]
    public FileDto(
        [JsonProperty("name")] String  name,
        [JsonProperty("kind")] DtoKind kind
    )
        : base( name: name, kind: kind )
    {
        if( kind != DtoKind.Directory ) throw new InvalidOperationException( "blargh" );
    }
}

Supposing I have a JSON array of FileSystemDto:

[
    { "name": "foo.txt", "kind": "File", "length": 12345 },
    { "name": "bar.txt", "kind": "File", "length": 12345 },
    { "name": "subdir", "kind": "Directory" },
]

I want Json.net to deserialize this to List<FileSystemDto>...

So define a subclass of DefaultContractResolver (or if you already have a resolver implementation then subclass (or compose) that) and override ResolveContractConverter:

public class MyContractResolver : DefaultContractResolver
{
    protected override JsonConverter? ResolveContractConverter( Type objectType )
    {
        if( objectType == typeof(FileSystemDto) )
        {
            return MyJsonConverter.Instance;
        }
        else if( objectType == typeof(FileDto ) )
        {
            // use default
        }
        else if( objectType == typeof(DirectoryDto) )
        {
            // use default
        }

        return base.ResolveContractConverter( objectType );
    }
}

Then implement MyJsonConverter:

public class MyJsonConverter : JsonConverter<FileSystemDto>
{
    public static MyJsonConverter Instance { get; } = new MyJsonConverter();

    private MyJsonConverter() {}

    // TODO: Override `CanWrite => false` and `WriteJson { throw; }` if you like.

    public override FileSystemDto? ReadJson( JsonReader reader, Type objectType, FileSystemDto? existingValue, Boolean hasExistingValue, JsonSerializer serializer )
    {
        if( reader.TokenType == JsonToken.Null ) return null;

        if( objectType == typeof(FileSystemDto) )
        {
            JObject jsonObject = JObject.Load( reader );
            if( jsonObject.Property( "kind" )?.Value is JValue jv && jv.Value is String kind )
            {
                if( kind == "File" )
                {
                    return jsonObject.ToObject<FileDto>( serializer );
                }
                else if( kind == "Directory" )
                {
                    return jsonObject.ToObject<DirectoryDto>( serializer );
                }
            }
        }

        return null; // or throw, depending on your strictness.
    }
}

Then, to deserialize, use a JsonSerializer instance with the ContractResolver set correctly, for example:

public static IReadOnlyList<FileSystemDto> DeserializeFileSystemJsonArray( String json )
{
    JsonSerializer jss = new JsonSerializer()
    {
        ContractResolver = new KuduDtoContractResolver()
    };

    using( StringReader strRdr = new StringReader( json ) )
    using( JsonTextReader jsonRdr = new JsonTextReader( strRdr ) )
    {
        List<FileSystemDto>? list = jss.Deserialize< List<FileSystemDto> >( jsonRdr );
        // TODO: Throw if `list` is null.
        return list;
    }
}