IF I need to invalidate the patent that is CIP.Taking priority from
In situations where priority is claimed to a document having different text, priority is determined on a claim-by-claim basis. That is, different claims of a CIP patent may have different priority dates, depending on when the corresponding subject matter was introduced.
So you have to go through one claim at a time, for example:
claim 1: contains some elements that were present in a parent filed Jan 1, 2006 and other elements that first appeared on July 2, 2006 => priority date is July 2, 2006.
claim 2 dependent on claim 1: additional elements of this claim were present on Jan 1, 2006. => priority date is still July 2, 2006 because the dependent claim includes all elements of its parent.
claim 3 dependent on claim 1: additional element was not introduced until Dec 3, 2006 => priority date is Dec 3, 2006.
You can invalidate some or all claims of a patent. The patent would only be regarded as invalidated when all its claims have been invalidated. A dependent claim can remain valid after its parent independent claim has been invalidated (which is a leading reason why patents contain so many dependent claims).
Please note that this discussion pertains to US patents only. Many other countries do not have a provision for CIP. Also please note that similar considerations apply where a patent claims priority to a provisional filing. Without examining the provisional, you don't really know whether the claimed priority date (filing date of provisional) is good, or whether the correct priority date for a particular claim should be the later filing date of the utility application.
And, sometimes it can be difficult to determine the proper priority date. For example an earlier document discloses a key concept of a claim but may not be sufficiently enabling, while a later document provides a fuller description.