I have a class
public class ItemList { public long Id { get; set; } public string Name { get; set; } public string Description { get; set; }
JSON is perfectly capable of expressing lists of integers, and the JSON you have posted is valid. You can simply separate the integers by commas:
{ "Id": "610", "Name": "15", "Description": "1.99", "ItemModList": [42, 47, 139] }