using System;
using System.Text.Json;
using System.Text.Json.Serialization;
using System.Reflection;


public class Program
{
    public static void Main()
    {
        var options = new JsonSerializerOptions();

        // works with System.Text.Json 8.x, throws with System.Text.Json 9.0.0
        // options.Converters.Add (new JsonStringEnumConverter (JsonNamingPolicy.SnakeCaseUpper)) ; // needs to preserve casing of Foo.A

        // also works for .NET 9
        options.Converters.Add(new CaseInsensitiveEnumConverterFactory(JsonNamingPolicy.SnakeCaseUpper));
        Console.WriteLine(JsonSerializer.Deserialize<Foo>("\"a\"", options));
    }
}

enum Foo
{
    A = 1,
}

public class CaseInsensitiveEnumConverterFactory : JsonConverterFactory
{
    private readonly JsonNamingPolicy? _jsonNamingPolicy;

    public CaseInsensitiveEnumConverterFactory()
    {

    }

    public CaseInsensitiveEnumConverterFactory(JsonNamingPolicy jsonNamingPolicy)
    {
        _jsonNamingPolicy = jsonNamingPolicy;
    }

    public override bool CanConvert(Type typeToConvert)
    {
        return typeToConvert.IsEnum;
    }

    public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)
    {
        return (JsonConverter)Activator.CreateInstance(
            typeof(CaseInsensitiveEnumConverter<>).MakeGenericType(typeToConvert),
            BindingFlags.Instance | BindingFlags.Public,
            null,
            [_jsonNamingPolicy],
            null)!;
    }
}

public class CaseInsensitiveEnumConverter<T> : JsonConverter<T> where T : struct, Enum
{
    private readonly JsonNamingPolicy? _jsonNamingPolicy;

    public CaseInsensitiveEnumConverter(JsonNamingPolicy? jsonNamingPolicy)
    {
        _jsonNamingPolicy = jsonNamingPolicy;
    }

    public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        if (reader.TokenType != JsonTokenType.String)
        {
            throw new JsonException();
        }

        var value = reader.GetString();
        if (Enum.TryParse(value, ignoreCase: true, out T result))
        {
            return result;
        }

        throw new JsonException($"Unable to convert \"{value}\" to Enum \"{typeof(T)}\".");
    }

    public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
    {
        var result = _jsonNamingPolicy != null ? _jsonNamingPolicy.ConvertName(value.ToString()) : value.ToString();

        writer.WriteStringValue(result);
    }
}