json-serialization-8.cs

internal class MyApiPostedValuesConverter : JsonConverter
{
    public override bool CanConvert(Type objectType) => objectType == typeof(IDictionary<string, IList<string>>);

    public override object ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
    {
        var properties = JToken.Load(reader).Cast<JProperty>().ToList();

        var result = new Dictionary<string, IList<string>>();
        foreach (JProperty property in properties)
        {
            List<string> values;
            if (property.Value.Type == JTokenType.Array)
            {
                values = ((JArray)property.Value)
                    .Select(ValueOrObjectToString)
                    .ToList();
            }
            else
            {
                values = new List<string> { property.Value.ToString() };
            }

            result.Add(property.Name, values);
        }

        return result;
    }

    private static string ValueOrObjectToString(JToken jtoken) =>
        jtoken is JValue jvalue
            ? jvalue.ToString()
            : ((JObject)jtoken).ToString(Formatting.None);

    public override bool CanWrite => false;

    public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer) => throw new NotImplementedException();
}