how Can set Expiry date for one Product Using C#

Well, if you are making the product you could go on and place a date into th ecode and hav it check the current date against the date in the code, if the date in the code has passed have the program not do anything. But that only works if you are releasing a beta that will work until say December 21, 2009 and is the same for everyone.

If you have a trial type of thing you will need to write a data file upon instalation that is protected in some way (so the user can't easily read/edit it) and then set the expiration date X number of days from the current date, again comparing dates whenever the program is used. You will probably also want to add in some fail safe so that if the data file can't be found the program doesn't do anything anyway, that way the user can't fool it into thinking it has more (infinite) time than it really does.

If you wanted you could also write the date to a computer registry or any other number of things.

I personally am not a C# programmer, but I believe the theory is the same.

BetaWar is correct.
But keep in mind that such techniques can easily be bypassed or removed by amateur reversers, so as long as it's an assignment question or a hobby project you should do fine with BetaWar's idea.