Defining the Design-time Interface
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Declaring part of a class as published makes that part public and also generates runtime type information. Among other things, runtime type information allows the Object Inspector to access properties and events.
Because they show up in the Object Inspector, the published parts of a class define that class's design-time interface. The design-time interface should include any aspects of the class that an application developer might want to customize at design time, but must exclude any properties that depend on specific information about the runtime environment.
Read-only properties cannot be part of the design-time interface because the application developer cannot assign values to them directly. Read-only properties should therefore be public, rather than published.