c# unittest list as inparam with no return

I am trying to unittest the code under, without any success. As you can see,

_dataObjectService.ParseDataObjects(listOfObjects, dataObjects, objectMode.New);

a list (listOfObjects) is used as an inparameter. The list is than set with values inside the method, (ParseDataObjects).

Tried to mock the service like this:

dataObjectServiceMock.Setup(m => m.ParseDataObjects(It.IsAny<Dictionary<ObjectMode,List<ObjectItem>>>(), _listUpdateObject, objectMode.New));


private Dictionary<ObjectMode, List<ObjectItem>> Test(Object object, Foo foo)
{
    var listOfObjects = new Dictionary<ObjectMode, List<ObjectItem>>();

    var dataObjects = _dataReader.GetDataObjects(foo.getObjects);
    var dataObjectItems = _dataReader.GetDataObjectItems(foo.getObjects);

    _dataObjectService.ParseDataObjects(listOfObjects, dataObjects, objectMode.New);
    _dataObjectService.ParseDataObjects(listOfObjects, dataObjectItems, objectMode.New);

    return listOfObjects ;
}

When I try to mock this I can mock _dataObjectService and the Test method, but I have no idea how to mock that list. listOfObjects is always empty.

How can I mock it?