using System;

using System.Text.Json;

using System.Text.Json.Serialization;

​

var myObject = new MyClass

{

    Prop1 = 10,

    Text = "String"

};

string json = JsonSerializer.Serialize(myObject, new JsonSerializerOptions { WriteIndented = true });

Console.WriteLine(json);

[JsonConverter(typeof(MyClassCustomConverter))]

public class MyClass

{

    public int Prop1 { get; set; }

    public string Text { get; set; }

}

​

public class MyClassCustomConverter : JsonConverter<MyClass>

{

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

    {

        throw new NotImplementedException();

    }

​

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

    {

        writer.WriteStartObject(); // {

        writer.WritePropertyName("SubObj");

        writer.WriteStartObject(); // {

        writer.WriteNumber(nameof(value.Prop1), value.Prop1);

        writer.WriteEndObject(); // }

        writer.WritePropertyName("SubObj2");

        writer.WriteStartObject(); // {

        writer.WriteString(nameof(value.Text), value.Text);

        writer.WriteEndObject(); // }

        writer.WriteEndObject(); // }

    }

}