using System;

using System.Diagnostics;

using System.Text.Json;

using System.Text.Json.Serialization;

public class Program

{

    public static void Main()

    {

        string serialized = JsonSerializer.Serialize(new ClassWithNullableStruct_ConverterOnProperty { MyStruct = new TestStruct { InnerValue = 5 } });

        Console.WriteLine(@"{""MyStruct"":{""InnerValue"":10}}" == serialized);

​

        ClassWithNullableStruct_ConverterOnProperty obj = JsonSerializer.Deserialize<ClassWithNullableStruct_ConverterOnProperty>(serialized);

        Console.WriteLine(15 == obj.MyStruct?.InnerValue);

    }

    private class ClassWithNullableStruct_ConverterOnProperty

    {

        [JsonConverter(typeof(JsonTestStructValueChangingConverter))]

        public TestStruct? MyStruct { get; set; }

    }

    private class JsonTestStructValueChangingConverter : JsonConverter<TestStruct>

    {

        public override bool CanConvert(Type typeToConvert) =>

            typeToConvert == typeof(TestStruct) || typeToConvert == typeof(TestStruct?);

​

        public override TestStruct Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)

        {

            Debug.Assert(reader.TokenType == JsonTokenType.StartObject);

            reader.Read();

​

            Debug.Assert(reader.TokenType == JsonTokenType.PropertyName);

            Debug.Assert(reader.GetString() == "InnerValue");

            reader.Read();

​

            var obj = new TestStruct

            {

                InnerValue = reader.GetInt32() + 5

            };

​

            reader.Read();

            Debug.Assert(reader.TokenType == JsonTokenType.EndObject);

​

            return obj;

        }

​

        public override void Write(Utf8JsonWriter writer, TestStruct value, JsonSerializerOptions options)

        {

            writer.WriteStartObject();

            writer.WriteNumber("InnerValue", value.InnerValue + 5);

            writer.WriteEndObject();

        }

    }

    private struct TestStruct

    {

        public int InnerValue { get; set; }

    }

}