Heya KyKyIIIKuH,
Welcome to our forums and thank-you for your interest in our network library.
ArrayList can not be supported as it is not a strongly typed object. The deserialiser must know exactly what the incoming type is otherwise it throws the error you are seeing. For the same reasons you could not send/receive a List<object>.
My recommendation would be to:
1. Use a strongly typed object.
or
2. Create a wrapper for your Arraylist and write a manual serialisation / serialisation step for the Arraylist part. We do a similar thing in this tutorial but using an image object instead.
If you have any further questions please feel free to post back.
Regards,
Marc