using System;

using System.Collections.Generic;

using Newtonsoft.Json;

​

public class Product

{

    public List<string> Sizes { get; set; } = new List<string> { "def" };

}

​

public class Program

{

    public static void Main()

    {

        Product product = new Product();

        product.Sizes = new List<string> { "Small" };

        string json = JsonConvert.SerializeObject(product);

        Console.WriteLine(json);

        Product product2 = JsonConvert.DeserializeObject<Product>(json);

        Console.WriteLine(product2.Sizes.Count);

    }

}