Server Session State is a design pattern used in web applications to maintain the state of a user’s session on the server side. This means that instead of storing the session state on the client side (e.g., in cookies), the state is stored on the server.

The Server Session State pattern is useful for a variety of reasons. First, it can improve the security of your application, as sensitive data is not stored on the client side where it can be easily accessed. Second, it can help to reduce the size of your client-side code, as you don’t need to store session data in cookies or other client-side storage mechanisms. Finally, it can help to ensure that session data is not lost if the user’s browser crashes or if they switch to a different device.

In C#, you can implement Server Session State using the built-in session object in ASP.NET. Here’s an example:
// Set session data
Session["username"] = "JohnDoe";
Session["cart"] = new List<string>() { "item1", "item2", "item3" };
// Get session data
string username = Session["username"] as string;
List<string> cart = Session["cart"] as List<string>;
// Remove session data

In this example, we set two pieces of session data: the user’s username and their shopping cart. We then retrieve the data and use it in our application. Finally, we remove the shopping cart data from the session. Note that the session data will persist until the session is explicitly ended or the session timeout is reached.

It’s important to keep in mind that the Server Session State pattern can have performance implications, as the session data must be stored and retrieved from the server on each request. It’s also important to properly manage the session data to prevent memory leaks and ensure that sensitive data is properly secured.