Deserialize JSON to multiple properties

Question

I am programming against a third party API which returns JSON data, but the format can be a little strange. Certain properties can either be an object (which contains an Id property), or a string (which is the Id of the object). For example, both of the following are valid:

{
    ChildObject: 'childobjectkey1'
}

and

{
    ChildObject: {
        Id: 'childobjectkey1',
        // (other properties)
    }
}

I'm trying to deserialize this using JSON.net into a strongly typed class, but haven't had much luck so far. My best idea was to serialise it to two properties, one a string and the other an object, and to use a custom JsonConverter for each to allow for the variable behaviour:

public abstract class BaseEntity
{
    public string Id { get; set; }
}

public class ChildObject : BaseEntity { }

public class MyObject
{
    [JsonProperty("ChildObject")]
    [JsonConverter(typeof(MyCustomIdConverter))]
    public string ChildObjectId { get; set; }

    [JsonProperty("ChildObject")]
    [JsonConverter(typeof(MyCustomObjectConverter))]
    public ChildObject ChildObject { get; set; }
}

However, setting the JsonProperty attribute on two properties with the same PropertyName causes the exception:

Newtonsoft.Json.JsonSerializationException: A member with the name 'ChildObject' already exists on '.....'. Use the JsonPropertyAttribute to specify another name.

I'm fairly sure the JsonConverter approach will work if I can get over this hurdle - I suspect the error is there because the JsonProperty attribute is used for Serialization as well as Deserialization. In this instance I have no interest in Serializing this class - it will only ever be used as the target for Deserialization.

I have no control over the remote end (it's a third party API), but I would like to be able to achieve this serialisation. I don't mind if it's using the approach I've started on, or one I've not thought of yet.

This question is also related, but there were no answers.

Answer 1

Rather than creating two separate converters for each of the fields, it would be wise to create a single converter for the "main" property and link the other one to it. ChildObjectId is derived from the ChildObject.

public class MyObject
{
    [JsonIgnore]
    public string ChildObjectId
    {
        get { return ChildObject.Id; }

        // I would advise against having a setter here
        // you should only allow changes through the object only
        set { ChildObject.Id = value; }
    }

    [JsonConverter(typeof(MyObjectChildObjectConverter))]
    public ChildObject ChildObject { get; set; }
}

Now to convert the ChildObject can be a bit of a challenge. There are two possible representations of the object: a string or an object. Determine what representation you have and perform the conversion.

public class MyObjectChildObjectConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(ChildObject);
    }

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

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var obj = serializer.Deserialize<JToken>(reader);
        switch (obj.Type)
        {
        case JTokenType.Object:
            return ReadAsObject(obj as JObject);
        case JTokenType.String:
            return ReadAsString((string)(JValue)obj);
        default:
            throw new JsonSerializationException("Unexpected token type");
        }
    }

    private object ReadAsObject(JObject obj)
    {
        return obj.ToObject<ChildObject>();
    }

    private object ReadAsString(string str)
    {
        // do a lookup for the actual object or whatever here
        return new ChildObject
        {
            Id = str,
        };
    }
}