Deserializing JSON when sometimes array and sometimes object

Question

I'm having a bit of trouble deserializing data returned from Facebook using the JSON.NET libraries.

The JSON returned from just a simple wall post looks like:

{     "attachment":{"description":""},     "permalink":"http://www.facebook.com/permalink.php?story_fbid=123456789" } 

The JSON returned for a photo looks like:

"attachment":{         "media":[             {                 "href":"http://www.facebook.com/photo.php?fbid=12345",                 "alt":"",                 "type":"photo",                 "src":"http://photos-b.ak.fbcdn.net/hphotos-ak-ash1/12345_s.jpg",                 "photo":{"aid":"1234","pid":"1234","fbid":"1234","owner":"1234","index":"12","width":"720","height":"482"}}         ], 

Everything works great and I have no problems. I've now come across a simple wall post from a mobile client with the following JSON, and deserialization now fails with this one single post:

"attachment":     {         "media":{},         "name":"",         "caption":"",         "description":"",         "properties":{},         "icon":"http://www.facebook.com/images/icons/mobile_app.gif",         "fb_object_type":""     }, "permalink":"http://www.facebook.com/1234" 

Here is the class I am deserializing as:

public class FacebookAttachment     {         public string Name { get; set; }         public string Description { get; set; }         public string Href { get; set; }         public FacebookPostType Fb_Object_Type { get; set; }         public string Fb_Object_Id { get; set; }          [JsonConverter(typeof(FacebookMediaJsonConverter))]         public List<FacebookMedia> { get; set; }          public string Permalink { get; set; }     } 

Without using the FacebookMediaJsonConverter, I get an error: Cannot deserialize JSON object into type 'System.Collections.Generic.List`1[FacebookMedia]'. which makes sense, since in the JSON, Media is not a collection.

I found this post which describes a similar problem, so I've attempted to go down this route: Deserialize JSON, sometimes value is an array, sometimes "" (blank string)

My converter looks like:

public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) {      if (reader.TokenType == JsonToken.StartArray)           return serializer.Deserialize<List<FacebookMedia>>(reader);      else           return null; } 

Which works fine, except I now get a new exception:

Inside JsonSerializerInternalReader.cs, CreateValueInternal(): Unexpected token while deserializing object: PropertyName

The value of reader.Value is "permalink". I can clearly see in the switch that there's no case for JsonToken.PropertyName.

Is there something I need to do differently in my converter? Thanks for any help.

Answer 1

A very detailed explanation on how to handle this case is available at "Using a Custom JsonConverter to fix bad JSON results".

To summarize, you can extend the default JSON.NET converter doing

1. Annotate the property with the issue

   [JsonConverter(typeof(SingleValueArrayConverter<OrderItem>))] public List<OrderItem> items; 

2. Extend the converter to return a list of your desired type even for a single object

   public class SingleValueArrayConverter<T> : JsonConverter {     public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)     {         throw new NotImplementedException();     }      public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)     {         object retVal = new Object();         if (reader.TokenType == JsonToken.StartObject)         {             T instance = (T)serializer.Deserialize(reader, typeof(T));             retVal = new List<T>() { instance };         } else if (reader.TokenType == JsonToken.StartArray) {             retVal = serializer.Deserialize(reader, objectType);         }         return retVal;     }      public override bool CanConvert(Type objectType)     {         return true;     } } 

As mentioned in the article this extension is not completely general but it works if you are fine with getting a list.