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.
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
Annotate the property with the issue
[JsonConverter(typeof(SingleValueArrayConverter<OrderItem>))]
public List<OrderItem> items;
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.