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; }     }