using System.Text.Json;

using System.Collections.Generic;

using System;

​

var json = @" {

""List"": []

}

";

var obj = JsonSerializer.Deserialize<Test>(json);

Console.WriteLine(obj.List.Count);

var json2 = JsonSerializer.Serialize(obj);

Console.WriteLine(json2);

class Test

{

    public List<int> List { get; set; }

}