using System;

using System.Collections.Generic;

​

using Newtonsoft.Json;

public class C {

    private readonly Dictionary<string, object> _props = new Dictionary<string, object>();

    public object this[string key] 

    {

        get { return _props[key];}

        set { _props[key] = value;}

    }

}

​

public class Program

{

    public static void Main()

    {

        var c = new C

        {

            ["sdf"] = new [] { "sdf", "sdf" }

        };

        Console.WriteLine(JsonConvert.SerializeObject(c));

    }

}