Which of the following are valid combinations of event visibility and handler method visibility?

In the context of event handling in object-oriented programming, understanding the visibility combinations of events and their corresponding handlers is crucial for maintaining encapsulation and proper access control.

A public event can be accessed and subscribed to from any scope, including external classes, which makes it suitable for broader usage. Likewise, a protected handler allows access to derived classes (subclasses). This combination ensures that while the event can be publicly accessed, the event handling logic remains contained within a safe scope, allowing only certain classes to define how the event is handled.

Using a public event with a protected handler method effectively balances flexibility with control, allowing the event to be widely utilized while still guarding the implementation of the handler method from outside interference. This ensures that only subclasses can intervene in the execution of the related logic, promoting a more secure design and protecting the integrity of the event's handling.

The other combinations either restrict accessibility in a way that doesn't match typical use cases or involve conflicting access levels, leading to limitations in usability or potential security issues. Understanding the correct balance of access permissions helps in designing robust and maintainable systems.