// See https://aka.ms/new-console-template for more information

using System.Text.Json;
using Dahomey.Json;
using Dahomey.Json.Attributes;

var json = """
            {
"MyProp": {
    "$type": "Foo",
    "Id": 1
}
}
""";

var jsonSerializerOptions = new JsonSerializerOptions();
jsonSerializerOptions.SetupExtensions();
var registry = jsonSerializerOptions.GetDiscriminatorConventionRegistry();

registry.RegisterAssembly(typeof(Encapsulator).Assembly);

var item = JsonSerializer.Deserialize<Encapsulator>(json, jsonSerializerOptions);

Console.WriteLine(item);

public class Encapsulator
{
    public AFoo MyProp { get; set; }
}

public abstract class AFoo
{
    public int Id { get; set; }
}

[JsonDiscriminator("Foo")]
public class Foo : AFoo
{

}