Well My custom rule involves checking if the Guid is NewGuid or Null. It fails if one of them is true. I have created a method for the same and then added it as instance rule.
Is there any other way in which this type of business rule can be used?
To My Surprise the rules work fine when the first instance is created using NewInvoice() Mehtod. But on all subsequent calls to NewInvoice() the rules are not checked