using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Collections.ObjectModel;

​

using System.Text.Json;

using System.Text.Json.Serialization;

using System.Text.Json.Serialization.Metadata;

using System.Text.Json.Nodes;

​

//https://stackoverflow.com/questions/77557549/configuring-which-type-that-fields-of-type-ilist-get-when-deserializing-json

​

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

​

class MyClass{

    public IList<NestedClass> MyList { get; set; }

}

​

class NestedClass{

    public IList<AnotherNestedClass> NestedList { get; set; }

}

​

class AnotherNestedClass{

    public string SomeValue { get; set; }

}

​

#nullable enable

​

    public class IListAsObservableCollectionConverter : JsonConverterFactory

    {

        public override bool CanConvert(Type typeToConvert) => 

            typeToConvert.IsInterface && typeToConvert.IsGenericType && typeToConvert.GetGenericTypeDefinition() == typeof(IList<>);

        public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options) => 

            (JsonConverter)Activator.CreateInstance(typeof(IListAsObservableCollectionConverterGeneric<>).MakeGenericType(typeToConvert.GetGenericArguments()))!;

        class IListAsObservableCollectionConverterGeneric<T> : JsonConverter<IList<T>>

        {

            public override void Write(Utf8JsonWriter writer, IList<T> value, JsonSerializerOptions options) =>

                JsonSerializer.Serialize(writer, value, value.GetType(), options);

            public override IList<T>? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) =>

                JsonSerializer.Deserialize<ObservableCollection<T>>(ref reader, options);

        }

    }

​

    class TestClass

    {

        public static void Test()

        {

            var expectedJson = 

                """

                {

                  "MyList": [

                    {

                      "NestedList": [

                        {

                          "SomeValue": "value 1"

                        },

                        {

                          "SomeValue": "value 2"

                        }

                      ]

                    }

                  ]

                }

                """;

            var options = new JsonSerializerOptions

            {

                Converters = { new IListAsObservableCollectionConverter() },

                // Other options as required, e.g.:

                IncludeFields = true, // Required to serialize & deserialize public fields as well as properties.

                WriteIndented = true,

            };

            var inputModel = new MyClass

            {

                MyList = new ObservableCollection<NestedClass>()

                {

                    new()

                    {

                        NestedList = new ObservableCollection<AnotherNestedClass>()

                        {

                            new() { SomeValue = "value 1" }, new() { SomeValue = "value 2" },

                        },

                    },

                },

            };

            var json = JsonSerializer.Serialize(inputModel, options);

            Console.WriteLine("Input JSON:");

            Console.WriteLine(json);

            var obj = JsonSerializer.Deserialize<MyClass>(json, options);

            Console.WriteLine(obj?.MyList?.GetType()); // This should be ObservableCollection<NestedClass>

​

            var json2 = JsonSerializer.Serialize(obj, options);

            Assert.AreEqual(typeof(ObservableCollection<NestedClass>), obj?.MyList?.GetType());

            Assert.That(obj!.MyList.Select(c => c?.NestedList?.GetType()).All(t => t == typeof(ObservableCollection<AnotherNestedClass>)));

            Assert.That(inputModel.MyList.SelectMany(c => c.NestedList).Select(c2 => c2.SomeValue).SequenceEqual(

                obj!.MyList.SelectMany(c => c.NestedList).Select(c2 => c2.SomeValue)));

            Assert.AreEqual(expectedJson, json);

            Assert.AreEqual(expectedJson, json2);

        }   

    }

​

    public class Program

    {

        public static void Main()

        {

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

            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;

            }

        }

    }

​