using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.IO;

using System.ComponentModel.DataAnnotations;

using System.Threading.Tasks;

​

using System.Text.Json;

using System.Text.Json.Serialization;

​

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

​

​

​

#nullable enable

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//https://stackoverflow.com/questions/75159862/performance-issue-with-deserlizing-large-json-file-into-objects-and-add-them-to

​

    public class Country

    {

        [Key]

        //[DatabaseGenerated(DatabaseGeneratedOption.None)]

        public int Id { get; set; }

        public string Name { get; set; }

        public string ISO2 { get; set; }

        public string ISO3 { get; set; }

​

        public ICollection<City> Cities { get; set; }

    }

​

    public class City

    {

        [Key]

        //[DatabaseGenerated(DatabaseGeneratedOption.None)]

        public int Id { get; set; }

        public string Name { get; set; }

        public int CountryId { get; set; }

        public Country Country { get; set; }        

    }

​

    class TestClass

    {

        public static async Task Test()

        {

            var fileName = "Question75159862.json";

            await File.WriteAllTextAsync(fileName, GetJson());

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            var countries = new List<Country>();

            await using var stream = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read, bufferSize: 4096, useAsync: true);

            var options = new JsonSerializerOptions

            {

                PropertyNamingPolicy = JsonNamingPolicy.CamelCase,

                //PropertyNameCaseInsensitive is required to deserialize "isO2" and "isO3" correctly

                PropertyNameCaseInsensitive = true,

            };

            await foreach (var country in JsonSerializer.DeserializeAsyncEnumerable<Country>(stream, options))

            {

                if (country == null)

                    continue;

                if (country.Cities != null)

                    foreach (var city in country.Cities)

                    {

                        // Do you need another instance of City?  If so you may clone the current one as follows:

                        var clone = JsonSerializer.Deserialize<City>(JsonSerializer.SerializeToUtf8Bytes(city));

​

                        city.CountryId = country.Id;

                        //context.Cities.Add(cities);

                        //context.Entry(cities).State = Microsoft.EntityFrameworkCore.EntityState.Detached;

                    }

                countries.Add(country);

            }

​

            Console.WriteLine("Deserialized and re-serialized {0}:", countries);

            var writeOptions = new JsonSerializerOptions

            {

                PropertyNamingPolicy = JsonNamingPolicy.CamelCase,

                PropertyNameCaseInsensitive = true,

                WriteIndented = true,

            };

            Console.WriteLine(JsonSerializer.Serialize(countries, writeOptions));

        }

        static string GetJson() =>

"""

[

   {

      "id":1,

      "name":"Afghanistan",

      "iso3":"AFG",

      "iso2":"AF",

      "cities":[

         {

            "id":141,

            "name":"‘Alāqahdārī Dīshū"

         },

         {

            "id":53,

            "name":"Aībak"

         },

         {

            "id":50,

            "name":"Andkhoy"

         },

         {

            "id":136,

            "name":"Āqchah"

         },

         {

            "id":137,

            "name":"Ārt Khwājah"

         },

         {

            "id":51,

            "name":"Asadabad"

         },

         {

            "id":52,

            "name":"Ashkāsham"

         },

         {

            "id":138,

            "name":"Āsmār"

         },

         {

            "id":54,

            "name":"Baghlān"

         },

         {

            "id":55,

            "name":"Balkh"

         }

      ]

   }

]

""";

    }

​

    public class Program

    {

        public static async Task Main(string[] args)

        {

            Console.WriteLine("Environment version: {0} ({1}), {2}", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , Environment.Version, Environment.OSVersion);

            Console.WriteLine("{0} version: {1}", typeof(JsonSerializer).Namespace, typeof(JsonSerializer).Assembly.FullName);

            Console.WriteLine();

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            try

            {

                await TestClass.Test();

            }

            catch (Exception ex)

            {

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

                Console.WriteLine(ex);

                throw;

            }

        }

    }

​