automatic polymorphic json.cs

using System;
using System.Linq;
using System.Text.Json;
using System.Text.Json.Serialization;
using System.Text.Json.Serialization.Metadata;

/* demo */

var options = JsonSerializerOptions.Web.WithPolymorphictypeInfoFor<Event>();

Event[] events = [new WebButtonEvent("123"), new HomeStateEvent("home/kitchen", "on")];

var serialized = JsonSerializer.Serialize(events, options);
Console.WriteLine(serialized);

var _events = JsonSerializer.Deserialize<Event[]>(serialized, options);
foreach (var _event in _events)
{
	Console.WriteLine(_event switch
	{
		WebButtonEvent button => $"webbuttonevent: {button.Id}",
		HomeStateEvent home => $"homestateevent: {home.Path} = {home.Value}",
		_ => "error"
	});
}

/* demo types */
record Event;
record WebButtonEvent(string Id) : Event;
record HomeStateEvent(string Path, string Value) : Event;


/* helper extensions */
public static class JsonExtensions
{
	public static JsonSerializerOptions WithPolymorphictypeInfoFor<T>(this JsonSerializerOptions options) => new(options)
	{
		TypeInfoResolver = new DefaultJsonTypeInfoResolver().WithAddedModifier(JsonExtensions.AddNativePolymorphicTypeInfoFor<Event>)
	};
	
	static void AddNativePolymorphicTypeInfoFor<T>(JsonTypeInfo jsonTypeInfo)
	{
		Type baseValueObjectType = typeof(T);
		if (jsonTypeInfo.Type == baseValueObjectType)
		{
			jsonTypeInfo.PolymorphismOptions = new JsonPolymorphismOptions
			{
				TypeDiscriminatorPropertyName = "type",
				IgnoreUnrecognizedTypeDiscriminators = true,
				UnknownDerivedTypeHandling = JsonUnknownDerivedTypeHandling.FailSerialization,
			};
			var types = AppDomain.CurrentDomain.GetAssemblies().SelectMany(a => a.GetTypes()).Where(t => t.IsSubclassOf(baseValueObjectType)); // Fixed MyBaseClass => ValueObject
			foreach (var t in types.Select(t => new JsonDerivedType(t, t.Name.ToLowerInvariant())))
			{
				jsonTypeInfo.PolymorphismOptions.DerivedTypes.Add(t);
			}
		}
	}
}