using System;

using System.Collections.Generic;

using System.Linq;

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

using Newtonsoft.Json.Serialization;

public class Program

{

    public static void Main()

    {

        string json = @"

            {

              ""user"": [

                {

                  ""key"": ""12345678"",

                  ""data"": [

                    {

                      ""id"": ""Name"",

                      ""string"": ""Bob""

                    },

                    {

                      ""id"": ""ElapsedTimeSinceLastMessage"",

                      ""timestamp"": 1618233964000

                    },

                    {

                      ""id"": ""Age"",

                      ""number"": 27

                    }

                  ]

                }

              ]

            }";

​

        var root = JsonConvert.DeserializeObject<RootObject>(json);

        foreach (User user in root.Users)

        {

            Console.WriteLine("User Key: " + user.Key);

            foreach (var kvp in user.Data)

            {

                Console.WriteLine(kvp.Key + ": " + kvp.Value);

            }

        }

    }

}

​

class RootObject

{

    [JsonProperty("user")]

    public List<User> Users { get; set; }

}

​

class User

{

    public string Key { get; set; }

    [JsonConverter(typeof(CustomDataConverter))]

    public Dictionary<string, object> Data { get; set; }

}

​

class CustomDataConverter : JsonConverter

{

    public override bool CanConvert(Type objectType)

    {

        return objectType == typeof(Dictionary<string, object>);

    }

​

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)

    {

        return JToken.Load(reader)

            .Children<JObject>()

            .ToDictionary(jo => (string)jo["id"],

                            jo => jo.Properties()

                                    .Where(jp => jp.Name != "id" && jp.Value is JValue)

                                    .Select(jp => ((JValue)jp.Value).Value)

                                    .FirstOrDefault());

    }

​

    public override bool CanWrite => false;

​

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)

    {

        throw new NotImplementedException();

    }

}

​