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.Threading;

using System.Runtime.CompilerServices;

​

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

using Newtonsoft.Json.Converters;

using Newtonsoft.Json.Serialization;    

​

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

​

#nullable enable

​

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

​

    public static partial class JsonExtensions

    {

        /// <summary>

        /// Asynchronously load and synchronously deserialize values from a stream containing a JSON array.  The root object of the JSON stream must in fact be an array, or an exception is thrown

        /// </summary>

        public static async IAsyncEnumerable<T?> DeserializeAsyncEnumerable<T>(Stream stream, JsonSerializerSettings? settings = default, [EnumeratorCancellation] CancellationToken cancellationToken = default)

        {

            var serializer = JsonSerializer.CreateDefault(settings);

            var loadSettings = new JsonLoadSettings { LineInfoHandling = LineInfoHandling.Ignore }; // For performance do not load line info.

            // StreamReader and JsonTextReader do not implement IAsyncDisposable so let the caller dispose the stream.

            using (var textReader = new StreamReader(stream, leaveOpen : true))

            using (var reader = new JsonTextReader(textReader) { CloseInput = false })

            {

                await foreach (var token in LoadAsyncEnumerable(reader, loadSettings, cancellationToken ).ConfigureAwait(false))

                    yield return token.ToObject<T>(serializer);

            }

        }

​

        /// <summary>

        /// Asynchronously load and return JToken values from a stream containing a JSON array.  The root object of the JSON stream must in fact be an array, or an exception is thrown

        /// </summary>

        public static async IAsyncEnumerable<JToken> LoadJTokenAsyncEnumerable(Stream stream, JsonLoadSettings? settings = default, [EnumeratorCancellation] CancellationToken cancellationToken = default)

        {

            // StreamReader and JsonTextReader do not implement IAsyncDisposable so let the caller dispose the stream.

            using (var textReader = new StreamReader(stream, leaveOpen : true))

            using (var reader = new JsonTextReader(textReader) { CloseInput = false })

            {

                await foreach (var token in LoadAsyncEnumerable(reader, settings, cancellationToken).ConfigureAwait(false))

                    yield return token;

            }

        }

​

        /// <summary>

        /// Asynchronously load and return JToken values from a stream containing a JSON array.  The root object of the JSON stream must in fact be an array, or an exception is thrown

        /// </summary>

        public static async IAsyncEnumerable<JToken> LoadAsyncEnumerable(JsonTextReader reader, JsonLoadSettings? settings = default, [EnumeratorCancellation] CancellationToken cancellationToken = default)

        {

            (await reader.MoveToContentAndAssertAsync().ConfigureAwait(false)).AssertTokenType(JsonToken.StartArray);

            cancellationToken.ThrowIfCancellationRequested();

            while ((await reader.ReadToContentAndAssert(cancellationToken).ConfigureAwait(false)).TokenType != JsonToken.EndArray)

            {

                cancellationToken.ThrowIfCancellationRequested();

                yield return await JToken.LoadAsync(reader, settings, cancellationToken).ConfigureAwait(false);

            }

            cancellationToken.ThrowIfCancellationRequested();

        }

​

        public static JsonReader AssertTokenType(this JsonReader reader, JsonToken tokenType) => 

            reader.TokenType == tokenType ? reader : throw new JsonSerializationException(string.Format("Unexpected token {0}, expected {1}", reader.TokenType, tokenType));

​

        public static async Task<JsonReader> ReadToContentAndAssert(this JsonReader reader, CancellationToken cancellationToken = default) =>

            await (await reader.ReadAndAssertAsync(cancellationToken).ConfigureAwait(false)).MoveToContentAndAssertAsync(cancellationToken).ConfigureAwait(false);

​

        public static async Task<JsonReader> MoveToContentAndAssertAsync(this JsonReader reader, CancellationToken cancellationToken = default)

        {

            if (reader == null)

                throw new ArgumentNullException();

            if (reader.TokenType == JsonToken.None)       // Skip past beginning of stream.

                await reader.ReadAndAssertAsync(cancellationToken).ConfigureAwait(false);

            while (reader.TokenType == JsonToken.Comment) // Skip past comments.

                await reader.ReadAndAssertAsync(cancellationToken).ConfigureAwait(false);

            return reader;

        }

​

        public static async Task<JsonReader> ReadAndAssertAsync(this JsonReader reader, CancellationToken cancellationToken = default)

        {

            if (reader == null)

                throw new ArgumentNullException();

            if (!await reader.ReadAsync(cancellationToken).ConfigureAwait(false))

                throw new JsonReaderException("Unexpected end of JSON stream.");

            return reader;

        }

    }

​

    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());

​

            var countries = new List<Country>();

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

            await foreach (var country in JsonExtensions.DeserializeAsyncEnumerable<Country>(stream))

            {

                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 = JToken.FromObject(city).ToObject<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);

            Console.WriteLine(JsonConvert.SerializeObject(countries, Formatting.Indented));

        }

        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();

​

            try

            {

                await TestClass.Test();

            }

            catch (Exception ex)

            {

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

                Console.WriteLine(ex);

                throw;

            }

        }

    }

​