using System.Text.Json;

using System.Text.Json.Serialization;

​

​

var json = """{"Test": "123", "Extra": "444"}""";

var output = JsonSerializer.Deserialize<Base>(json);

System.Console.WriteLine(output is More);

System.Console.WriteLine(output is Base);

​

System.Console.WriteLine(output.GetType().GetProperties().ToString());

​

[JsonDerivedType(typeof(More), typeDiscriminator: "withExtra")]

public class Base 

{

    public string Test { get; set; }

}

​

public class More : Base 

{

    public string Extra { get; set; }

}