Potential for flattening an abstract object into JSON for serialization/deserialization?

FlattenAbstractObject.cs

public class Test
{

	public int Id {get;set;}
	[JsonIgnore] public Foo {get;set;} = new Foo();
	public BigInteger Bar
	{
		get => Foo.Bar;
		set => Foo.Bar = value;
	}
	
	private string _propA;
	[JsonProperty(PropertyValue = "propA")]
	public string PropA
	{
		get
		{
			if (Foo.Baz is BazA a)
			{
				return a.PropA;
			}
			
			return null;
		}
		set
		{
			_propA = value;
			
			// && whatever other properties are not null/not default that
			// decide on the concrete type
			if (_propA != null )
			{
				Foo.Baz = new BazA()
				{
					PropA = _propA,
					// other properties
				}
			}
		}
	}

	private string _propB;
	[JsonProperty(PropertyValue = "propB")]
	public string PropB
	{
		get
		{
			if (Foo.Baz is BazB b)
			{
				return b.PropB;
			}
			
			return null;
		}
		set
		{
			_propB = value;
			
			// && whatever other properties are not null/not default that
			// decide on the concrete type
			if (_propB != null ) 
			{
				Foo.Baz = new BazB()
				{
					PropB = _propB,
					// other properties
				}
			}
		}
	}	
	
}

public class Foo
{
	public BigInteger Bar {get;set;}
	public BazBase Baz {get;set;}
}

public abstract class BazBase
{
}

public class BazA : BazBase
{
	public string PropA {get;set;}
}

public class BazB : BazBase
{
	public string PropB {get;set;}
}