Castor and collections: mapping file needed for unmarshalling?

[Chris]

I am using Castor to marshal and unmharshal some objects.

I have an object that has a List of strings.
I can marshal it without using a mapping file.
However, when I try to unmarshal it without using a mapping file,
I get an error that the list elements occur more than once.
If I use a mapping file and indicate that the data member is
a collection, it unmarshals without error.

My question is:
Am I *required* to provide a mapping file whenever the class
to be marshalled/unmarshalled contains a collection of some sort,
or can I configure Castor to implicity recognize/allow collections
either programmatically or through its properties file?

Thanks in advance