Deserializing descendent objects

Question

I'm communicating with a JSON-based API which I can't change. It always returns a Response object with a varying Result object inside. Typically it looks like this:

{ "ver": "2.0", "result": { "code": 0 } }

For certain commands the Result object is 'grown' by adding extra properties:

{ "ver": "2.0", "result": { "code": 0, "hostName": "sample", "hostPort": 5000 } }

I've used Newtonsoft attributes to define the objects as follows:

 internal class RpcResponse
    {
        [JsonProperty(PropertyName = "ver")]
        public string Version { get; set; }

        [JsonProperty(PropertyName = "result")]
        public RpcResponseResult Result
        {
            get;
            set;
        }

internal class RpcResponseResult
    {
        [JsonProperty(PropertyName = "code")]
        public int Code { get; set; }
    }

internal class RpcExtendedResponseResult: RpcResponseResult
    {
        [JsonProperty(PropertyName = "hostName")]
        public string HostName { get; set; }

        [JsonProperty(PropertyName = "hostPort")]
        public int HostPort { get; set; }

But when the Response object is deserialized:

RpcResponse rspResponse = JsonConvert.DeserializeObject<RpcResponse>(rspString);

Its Result property always appears as an RpcResponseResult object, ie. JsonConvert doesn't know to construct it as a RpcExtendedResponseResult object.

Is there some way with Attributes or Converters to reinstate the correct descendent object? I feel like I'm missing something obvious!

Are you lookin for something like this http://stackoverflow.com/questions/3142495/deserialize-json-into-c-sharp-dynamic-object — Justin CI, Mar 17 '15 at 06:45

score 1 · Accepted Answer · answered Mar 17 '15 at 04:33

It's because the type of the object is RpcResponseResult. The deserializer can only deserialize fields that are declared in the type of the field specified. It can't determine because a class has "hostName" its now an RpcExtendedResponseResult.

If I were doing this, I might make the result a container for all possible fields with default values if needed, and then you can fill another object as needed.

internal class RpcResponseResultContainer
{
    [JsonProperty(PropertyName = "code")]
    public int Code { get; set; }

    [JsonProperty(PropertyName = "hostName")]
    private string mHostName = string.Empty;
    public string HostName 
    { 
       get { return mHostName;} 
       set { mHostName = value; }
    }

    [JsonProperty(PropertyName = "hostPort")]
    private int mHostPort = -1;
    public int HostPort 
    { 
       get { return mHostPort;} 
       set { mHostPort = value;}
    }

Then if you really wanted to get your object as you want it, you could do something like this in your container class:

 public RpcResponseResult GetActualResponseType()
 {
     if(HostPort != -1 && !string.IsNullOrEmtpy(HostName))
     {
         return new RpcExtendedResponseResult() { Code = this.Code, HostName = this.HostName, HostPort = this.HostPort};
     }
     return new RpcResponseResult() { Code = this.Code };
 }

score 0 · Answer 2 · edited May 23 '17 at 11:58

First, credit to Matthew Frontino for providing the only answer which I've accepted.

However I opted not to make a single result container, so here's what I ended up doing.

First I started with this page: How to implement custom JsonConverter in JSON.NET to deserialize a List of base class objects?
I used the version of JsonCreationConverter provided there by Alain.

I added the CanWrite override as suggested there by Dribbel:

public override bool CanWrite
{
    get { return false; }
}

I also added my own helper function to JsonCreationConverter:

protected bool FieldExists(string fieldName, JObject jObject)    {
    return jObject[fieldName] != null;
}

Then I created my own converter as follows:

class RpcResponseResultConverter : JsonCreationConverter<RpcResponseResult>
{
    protected override RpcResponseResult Create(Type objectType, JObject jObject)
    {
        // determine extended responses
        if (FieldExists("hostName", jObject) &&
            FieldExists("hostPort", jObject) )
        {
            return new RpcExtendedResponseResult();
        }
        //default
        return new RpcResponseResult();
    }
}

Then I deserialize the top-level class and supply any converters to be used. In this case I only supplied one, which was for the nested class in question:

RpcResponse rspResponse = JsonConvert.DeserializeObject<RpcResponse>(
    rspString, 
    new JsonSerializerSettings {
        DateParseHandling = Newtonsoft.Json.DateParseHandling.None,
        Converters = new List<JsonConverter>( new JsonConverter[] {
                new RpcResponseResultConverter()
                })
        });

Notes:

Anything not explicitly handled by a converter (such as the top-level class) is deserialized using the default converter built into JsonConvert.
This only works if you can identify a unique set of fields for every descendent class.

Deserializing descendent objects

2 Answers2