public override IEnumerable<double> Convert(object o)
{
// check for valid cateogry list
if (Categories == null || Categories.Length == 0)
throw new InvalidOperationException(string.Format("{0} categories do not exist.", Name));
for (int i = 0; i < Categories.Length; i++)
{
if (Categories[i] == (o as Guid?))
{
yield return i;
}
}
}