using System;

using Newtonsoft.Json;

using System.Collections.Generic;

​

class Foo

{

    public int Key { get; set; }

    public int Value { get; set; }

}

public class Program

{

    public static void Main()

    {

​

        var xs = GetItems(new []{ 1, 2, 3});

​

        foreach(var x in xs) {

            x.Value = x.Key + 1;

        }

​

        var json = JsonConvert.SerializeObject(xs, Formatting.Indented);

​

        Console.WriteLine(json);

​

    }

    static IEnumerable<Foo> GetItems(IEnumerable<int> vals) {

        foreach(var v in vals) {

            yield return new Foo { Key = v };

        }

    }

}