ILookup serializer for json.net

LookupSerializer.cs

    using System;
    using System.Linq;
    using Newtonsoft.Json;
    using Newtonsoft.Json.Linq;
    
    public class LookupSerializer
    {
        public static LookupSerializer<Key, Value> New<Key, Value>(ILookup<Key,Value> lookup)
        {
            return new LookupSerializer<Key, Value>();
        }
    }
  
     public class LookupSerializer<Key, Value> : JsonConverter
    {
        public override bool CanConvert(Type objectType)
        {
            var result = objectType.GetInterfaces()
                .Any(a => a.IsGenericType && a == typeof(ILookup<Key, Value>));
            return result;
        }

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            throw new NotImplementedException();
        }

        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
        {
            var target = value as ILookup<Key, Value>;
            var obj = new JObject();
            var keys = target
                .Select(a => a.Key)
                .Distinct();
            
            foreach(var key in keys)
            {
                obj.Add(key.ToString(), JArray.FromObject(target[key]));
            }
            obj.WriteTo(writer);
        }
    }

You can use this like LookupSerializer.New(YourILookupHere) or you can do new LookupSerializer<YourILookupKey, YourILookupValue>();

and then pass it into Newtonsoft.Json aka Json.Net as a custom serializer