Defining the Runtime Interface
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Declaring part of a class as public makes that part visible to any code that has access to the class as a whole.
Public parts are available at run time to all code, so the public parts of a class define its runtime interface. The runtime interface is useful for items that are not meaningful or appropriate at design time, such as properties that depend on runtime input or which are read-only. Methods that you intend for application developers to call must also be public.