C#.NET — Which statements about interfaces are correct? 1) An interface can contain properties, methods, and events. 2) The keyword 'must implement' forces implementation of an interface. 3) Interfaces can be overloaded. 4) Interfaces can be implemented by a class or a struct. 5) Enhanced implementations of an interface can be developed without breaking existing code (by adding new implementing types without changing the interface).

Introduction / Context:
Interfaces in C#.NET define contracts that types agree to fulfill. This question checks core facts: what members an interface may declare, who can implement it, and how adding implementations affects existing code.

Given Data / Assumptions:

• C# interface syntax and capabilities are considered in the classic sense (properties, methods, events, indexers).
• We assume no Visual Basic keywords (like MustInherit/Implements) in C#.
• “Enhanced implementations without breaking code” refers to adding new implementing types while keeping the interface unchanged.

Concept / Approach:
Interfaces can declare members such as methods, properties, events, and indexers. Both classes and structs can implement interfaces. Adding new implementations (new classes/structs that implement the same interface) does not break existing consumers because existing binaries compile against the unchanged interface contract. In contrast, changing the interface (e.g., adding new required members) can break implementers.

Step-by-Step Solution:

Verification / Alternative check:
Define an interface with properties/methods/events; implement it in a class and a struct; add another class that implements it later. Existing code continues to work.

Why Other Options Are Wrong:

• Option A includes a false statement (2).
• Option C includes statement 3 (false) and omits 1 and 5.
• Option D only keeps the false statement 3.
• Option E is invalid because there is a correct set: 1, 4, 5.

Common Pitfalls:
Confusing VB keywords with C#. Also, assuming you can change interface members freely without breaking existing implementers.

Final Answer:
1, 4, 5