private void ProcessContents(List<Asset> contents)
{
    contents.ForEach((current) =>
    {
        if (current.Contents != null)
            ProcessContents(current.Contents);
        else
        {
            // _api = new EveApi(); at class scope
            var pt = _api.EveApiCore.FindProductType(current.TypeID);
            string name = pt.Name;
            // do some stuff with the current item/name
        }
    });
}