'Session' does not exist in the current context

You either need to convert your class to a Page by inheriting from Page, or have the Session passed in, or use HttpContext.Current.Session.

Use

if (HttpContext.Current == null || 
    HttpContext.Current.Session == null || 
    HttpContext.Current.Session["ShoppingCart"] == null)

instead of

if (Session["ShoppingCart"] == null)

The issue is that your class does not inherit from Page. you need to Change

public class ShoppingCart

public class ShoppingCart : Page

and it will work

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