C_SHARP_SKILLS/.ai/skills/07-interfaces-generics-events-inheritance.md

Skill 07 - Interfaces, Generics, Events, Operators, and Inheritance

Interfaces

• Use interfaces to model capabilities or contracts, not every class by default.
• Keep interfaces small and cohesive.
• Prefer dependency inversion at boundaries: data stores, clocks, HTTP clients, external services, email, files, and message queues.
• Do not create one-to-one interfaces for every implementation unless mocking, plugin replacement, or architecture requires it.

Generics

• Use generics for type-safe reuse.
• Add constraints (where T : ...) to express required capabilities.
• Prefer IEnumerable<T> for simple read-only enumeration, IReadOnlyList<T> when count/index matters, and concrete collections when mutation is required internally.

Events and Delegates

• Use events for notifications from one object to many subscribers.
• Prefer standard EventHandler / EventHandler<TEventArgs> unless a custom delegate adds clarity.
• Avoid memory leaks by unsubscribing long-lived event subscriptions.

Operator Overloading

• Overload operators only when they match natural domain meaning.
• Keep overloaded operators unsurprising and consistent with equality/comparison.

Inheritance

• Prefer composition over inheritance for code reuse.
• Use inheritance for true “is-a” relationships.
• Mark base members virtual only when overriding is intended.
• Use override deliberately and keep base class contracts intact.
• Use sealed for classes or overrides that should not be extended.
• Use pattern matching or safe casts rather than unsafe casts.

Extension Members

• Use extension methods to add convenience behavior to types you do not own.
• Keep extension methods pure and obvious.
• Do not hide expensive I/O behind innocent-looking extension methods.