gistfile1.txt

using System;
using System.Collections.Generic;
using System.Web.Script.Serialization;

namespace TestApp
{
    class Program
    {
        static void Main()
        {
            string json = "{ \"expiry\":1551354842.0 }";
            var jsonSerializer = new JavaScriptSerializer();
            var data = jsonSerializer.Deserialize<Dictionary<string, object>>(json);

            object exp;
            data.TryGetValue("expiry", out exp);
            Console.WriteLine(exp);
            Console.WriteLine(exp.GetType());

            var tempExpiry = long.Parse(exp.ToString());
            Console.WriteLine(tempExpiry);
        }
    }
}