using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Runtime.Serialization.Formatters;

using System.IO;

using System.Data;

using System.ComponentModel.DataAnnotations;

using System.Globalization;

using System.Reflection;

using System.Text.Json;

using System.Text.Json.Serialization;

​

using NUnit.Framework; // Throws NUnit.Framework.AssertionException

​

//https://stackoverflow.com/questions/58997718/modifying-a-json-file-using-system-text-json

​

    class TestClass

    {

        public static void Test()

        {

            var newJson = TestMethod();

            Console.WriteLine("\nNew users only: ");

            Console.WriteLine(newJson());

        }

        public static Func<string> TestMethod()

        {

            ICollection<long> existingUserIds;

            using(var existingUsersDocument = JsonDocument.Parse(GetExistingUsersJson()))

            {

                existingUserIds = existingUsersDocument.RootElement.EnumerateArray().Select(u => u.GetProperty("id").GetInt64()).ToHashSet();

            }

            Console.Write("\nExisting user ids:");

            Console.WriteLine(JsonSerializer.Serialize(existingUserIds));

            Predicate<long> shouldSkip = i => existingUserIds.Contains(i);

            var rawJsonDownload = GetRawJsonDownload();

            using var usersDocument = JsonDocument.Parse(rawJsonDownload);

            var users = usersDocument.RootElement.EnumerateArray()

                .Where(e => !shouldSkip(e.GetProperty("id").GetInt64()))

                .Select(e => e.Clone())

                .ToList();

            return Json(users);

        }   

        public static Func<string> Json<T>(T obj)

        {

            return () => JsonSerializer.Serialize(obj, new JsonSerializerOptions { WriteIndented = true });

        }

        static string GetRawJsonDownload()

        {

            var json = @"[

   {

      ""id"":3,

      ""name"":""name 3"",

      ""address"" : { ""Line1"" : ""line 1"", ""Line2"" : ""line 2""},

      ""discount"" : 123.1212121121212121

   },

   {

      ""id"":4,

      ""name"":""name 4"",

      ""address"" : { ""Line1"" : ""line 1"", ""Line2"" : ""line 2""},

      ""discount"" : 22.220000000000000000000

   },

   {

      ""id"":5,

      ""name"":""name 5"",

      ""address"" : { ""Line1"" : ""line 1"", ""Line2"" : ""line 2""},

      ""discount"" : 123.1212121121212121

   }

]";

            return json;

        }

        static string GetExistingUsersJson()

        {

            var json = @"[

   {

      ""id"":1,

      ""name"":""name 1"",

      ""address"" : { ""Line1"" : ""line 1"", ""Line2"" : ""line 2""},

      ""discount"" : 10.1100000000000000000000000

   },

   {

      ""id"":2,

      ""name"":""name 2"",

      ""address"" : { ""Line1"" : ""line 1"", ""Line2"" : ""line 2""},

      ""discount"" : 123.1212121121212121

   },

   {

      ""id"":3,

      ""name"":""name 3"",

      ""address"" : { ""Line1"" : ""line 1"", ""Line2"" : ""line 2""},

      ""discount"" : 123.1212121121212121

   }

]";

            return json;

        }

    }

​

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Environment version: {0} ({1})", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , GetNetCoreVersion());

            Console.WriteLine("System.Text.Json version: " + typeof(JsonSerializer).Assembly.FullName);

            Console.WriteLine();

​

            try

            {

                TestClass.Test();

            }

            catch (Exception ex)

            {

                Console.WriteLine("Failed with unhandled exception: ");

                Console.WriteLine(ex);

                throw;

            }

        }

        public static string GetNetCoreVersion()

        {

            //https://techblog.dorogin.com/how-to-detect-net-core-version-in-runtime-ecd65ad695be

            //

            var assembly = typeof(System.Runtime.GCSettings).GetTypeInfo().Assembly;

            var assemblyPath = assembly.CodeBase.Split(new[] { '/', '\\' }, StringSplitOptions.RemoveEmptyEntries);

            int netCoreAppIndex = Array.IndexOf(assemblyPath, "Microsoft.NETCore.App");

            if (netCoreAppIndex > 0 && netCoreAppIndex < assemblyPath.Length - 2)

                return assemblyPath[netCoreAppIndex + 1];

            return null;

        }

    }

​