Using Newtonsoft.Json, how do I deserialize JSON to a type with an IEnumerable property containing an interface type?

Question

I am consuming a RESTful Web service sending JSON, that I try to deserialize using HttpContent.ReadAsAsync<T>. The type I try to deserialize to declares a property that returns an IEnumerable containing an interface type. This code snippet demonstrates the kind of type I'm trying to deserialize to:

public class Data
{
    public IEnumerable<IChild> Children { get; set; };
}

The problem is that Newtonsoft.Json, underlying HttpContent.ReadAsAsync<T> doesn't understand how to deserialize objects of type IChild, the latter being an interface. How can I specify to Newtonsoft.Json how to deserialize IChild to a concrete type?

@L.B Does it matter in this case? The problem is just how to tell Newtonsoft.Json to convert the interface (IChild) to a concrete type, f.e.x Child. — aknuds1, Aug 30 '12 at 13:32

score 4 · Accepted Answer · answered Aug 30 '12 at 13:39

You can use a custom Converter to tell JSON.NET how to deserialize types of that interface. The code below shows an example.

public class StackOverflow_12197892
{
    public class Data
    {
        public IEnumerable<IChild> Children { get; set; }

        public override string ToString()
        {
            return string.Format("Data{{Children=[{0}]}}",
                string.Join(", ", Children.Select(c => string.Format("{0}/{1}", c.Name, c.IsFemale ? "girl" : "boy"))));
        }
    }
    public interface IChild
    {
        string Name { get; }
        bool IsFemale { get; }
    }
    public class Son : IChild
    {
        public Son(string name)
        {
            this.Name = name;
        }

        public string Name { get; private set; }
        public bool IsFemale { get { return false; } }
    }
    public class Daughter : IChild
    {
        public Daughter(string name)
        {
            this.Name = name;
        }

        public string Name { get; private set; }
        public bool IsFemale { get { return true; } }
    }
    class ChildConverter : Newtonsoft.Json.JsonConverter
    {
        public override bool CanConvert(Type objectType)
        {
            return typeof(IChild).IsAssignableFrom(objectType);
        }

        public override object ReadJson(Newtonsoft.Json.JsonReader reader, Type objectType, object existingValue, Newtonsoft.Json.JsonSerializer serializer)
        {
            JObject obj = serializer.Deserialize<JToken>(reader) as JObject;
            if (obj != null)
            {
                bool isFemale = obj["isFemale"].ToObject<bool>();
                string name = obj["name"].ToObject<string>();
                if (isFemale)
                {
                    return new Daughter(name);
                }
                else
                {
                    return new Son(name);
                }
            }
            else
            {
                return null;
            }
        }

        public override void WriteJson(Newtonsoft.Json.JsonWriter writer, object value, Newtonsoft.Json.JsonSerializer serializer)
        {
            throw new NotImplementedException();
        }
    }
    public static void Test()
    {
        Newtonsoft.Json.JsonSerializer serializer = new Newtonsoft.Json.JsonSerializer();
        serializer.Converters.Add(new ChildConverter());
        string json = "{'Children':[{'name':'John',isFemale:false},{'name':'Mary',isFemale:true}]}".Replace('\'', '\"');
        var obj = serializer.Deserialize(new StringReader(json), typeof(Data));
        Console.WriteLine(obj);
    }
}

Thanks. How would you integrate the converter with `HttpContent.ReadAsAsync` though, as this calls Newtonsoft.Json indirectly? — aknuds1, Aug 30 '12 at 13:54
You'd need to set the converter in the formatter on the content. — carlosfigueira, Aug 30 '12 at 14:15
I configured the converter by setting `[ConverterAttribute]` on `IChild`. — aknuds1, Aug 30 '12 at 14:24

score 0 · Answer 2 · answered Jan 14 '14 at 09:52

It is quite simple and out of the box support provided by json.net, you just have to use the following JsonSettings while serializing and Deserializing:

JsonConvert.SerializeObject(graph,Formatting.None, new JsonSerializerSettings()
                                                                                               {
                                                                                                 TypeNameHandling = TypeNameHandling.Objects,
                                                                                               TypeNameAssemblyFormat = System.Runtime.Serialization.Formatters.FormatterAssemblyStyle.Simple
                                                                                           });

and for Deserialzing use the below code:

JsonConvert.DeserializeObject(Encoding.UTF8.GetString(bData),type,
                                                       new JsonSerializerSettings()
                                                           {TypeNameHandling = TypeNameHandling.Objects});

Just take a note of the JsonSerializerSettings object initializer, that is important for you.

Using Newtonsoft.Json, how do I deserialize JSON to a type with an IEnumerable property containing an interface type?

2 Answers2

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