c# - 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.