Hands-On Design Patterns with C# and .NET Core

Observer pattern

An interesting pattern to explore in more detail is the Observer pattern. This pattern allows for instances to be informed of when a particular event happens in another instance. In this way, there are many observers and a single subject. The following diagram illustrates this pattern:

Let's provide an example by creating a simple C# console application that will create a single instance of a Subject class and multiple Observer instances. When a quantity value changes in the Subject class, we want each Observer instance to be notified.

The Subject class contains a private quantity field that is updated by a public UpdateQuantity ...