The observer pattern is a design pattern, largely popularized by the Gang of Four, which allows objects to receive notifications from another object. Imagine a newsletter signup page, where you can signup for various newsletters that are broadcasted by email. There needs to be the ability to signup multiple users for one newsletter; after all, it would be a shame to create only a one-to-one relationship. Rather, there needs to be a one-to-many relationship, with the many part being an open-ended amount of people.

It is possible to set this up in a .NET application as well. A subject (the newsletter) has one or many observers (newsletter subscribers) of that subject. Whenever the subject needs to broadcast some sort of change, all of the observers get notified of that change. In the newsletter example, when a newsletter changes its content, it is broadcasted to the subscribers. This can be more difficult to setup in ASP.NET because of some of the limitations that occur with the stateless environment. However, it is possible and this article will show you how.