I pointed out two methods, you found a third.
There's a difference between a willingness to help and doing the work for someone.
A search on Google for 'LastIndexOf C#' brings back a host of examples, some virtually identical to your problem. Why should I write yet another example because you couldn't be bothered to do a quick search once you'd been given a pointer?
I don't normally get annoyed but when you've answered over 2000 questions on a forum then you can start criticising others attitude.
Joe (Microsoft MVP - XML