using System;

using System.Text.Json;

public class Program

{

    public static void Main()

    {

        var input =  new MyClass

                     {

                        InnerMember = new InnerClass() { Data =  null }

                     };

        var objString = JsonSerializer.Serialize(input);

        var doc = JsonDocument.Parse(objString);

        var root = doc.RootElement;

        root.ValueKind.Dump();

        root.GetProperty("InnerMember").GetProperty("Data").ValueKind .Dump();

    }

    public class MyClass {

        public InnerClass InnerMember {get;set;}

    }

    public class InnerClass{    

        public FooBar Data {get;set;}

    }

    public class FooBar{    

        public int Foo {get;set;}

        public string Bar {get;set;}

    }

}