fsharp union codec

codec.cs

public static class FSharpUnionCodecBuilder
    {
        public delegate T Decoder<T>(byte[] bytes, string subType);
        public delegate byte[] Encoder<T>(T input);
        
        public static Tuple<Decoder<T>,Encoder<T>> Build<T>(T unionType) where T : class
        {
            var unionCaseFields = FSharpType.GetUnionCases(typeof(T), BindingFlags.Public)
                .ToDictionary(uc => $"{uc.DeclaringType.Name}:{uc.Name}",
                    uc => new Tuple<UnionCaseInfo, Type>(uc,
                        uc.GetFields().FirstOrDefault()?.DeclaringType 
                            ?? throw new ArgumentException("Must have union case argument")));

            var decoder = (Decoder<T>) ((byte[] bytes, string subTypeName) => {
                var jsonText = System.Text.Encoding.UTF8.GetString(bytes);
                var (targetSubType, fieldType) =
                    unionCaseFields[subTypeName] ?? throw new ArgumentException("Valid subtype expected");
                var subInstance = JsonConvert.DeserializeObject(jsonText, fieldType);
                var wrapped = FSharpValue.MakeUnion(targetSubType, new [] { fieldType }, null);
                return (T) wrapped;
            });

            Encoder<T> encoder = null; //TODO
            
            return new Tuple<Decoder<T>, Encoder<T>>(decoder, encoder);
        }
    }