using System;

using System.Linq;

using System.Collections.Generic;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        Dictionary<int, Foo> dict = new Dictionary<int, Foo>();

        dict.Add(1, new Foo{

            Prop1 = "one",

            Prop2 = "oneValue"

        });

        dict.Add(3, new Foo{

            Prop1 = "three",

            Prop2 = "threeValue"

        });

        List<TestClass> list = new List<TestClass>();

        foreach(KeyValuePair<int, Foo> kv in dict){

            list.Add(new TestClass(){

                i = kv.Key,

                someProp = kv.Value.Prop1

            });

        }

        var json = JsonConvert.SerializeObject(list);

        Console.WriteLine(json);

    }

}

​

public class TestClass{

    public int i{get;set;}

    public string someProp{get;set;}

}

​

public class Foo{

    public string Prop1{get;set;}

    public string Prop2{get;set;}

}