The Pair class is used as a basic structure to store two related objects. It is a utility class that is used in various ways throughout ASP.NET, such as during page state management tasks or in configuration section handlers. You can use the Pair class in your own code anywhere that you need a structure to contain two related objects and where data hiding is not essential. The Pair class does not encapsulate its object references, First and Second, in properties; it exposes them directly to all calling code as public class fields.
The Pair class can be used in several ways in page state persistence implementations. The most common use is as a container for both the ViewState and ControlState collections. In this case, the First property is used for ViewState, and the Second for ControlState.