# 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.