using System;

using System.Text.Json;

public class Program

{

    public static void Main()

    {

        var data = new Data<ExtendedSubData>

        {

            Prop1 = "1",

            SubData = new ExtendedSubData

            {

                Prop2 = "2",

                Prop3 = "3"

            }

        };

​

        var json = SerializData(data);

        Console.WriteLine(json);

    }

    private static string SerializData<TSubData>(Data<TSubData> data)

        where TSubData : SubData

    {

        return JsonSerializer.Serialize<object>(data);

    }

}

​

public class Data<TSubData>

    where TSubData : SubData

{

    public string Prop1 { get; set; }

    public TSubData SubData { get; set; }

}

​

public abstract class SubData

{

    public string Prop2 { get; set; }

}

​

public class ExtendedSubData : SubData

{

    public string Prop3 { get; set; }

}