using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        string json = @"

            {

              ""Accounts"": [ 1, 2, 3 ]

            }";

        var config = Newtonsoft.Json.JsonConvert.DeserializeObject<Config>(json);

        Console.WriteLine(string.Join(", ", config.Accounts));

        Console.WriteLine(config.Accounts.GetType().FullName);

    }

}

​

public class Config

{

    public IReadOnlyCollection<int> Accounts { get; } = new HashSet<int>();

}

​