Update AV1130: Return interfaces to unchangeable collections

[dennisdoomen]

This PR updates guideline AV1130.

It was split out of #298 so the change can be reviewed independently.

Files:

• _rules/1130.md

Part of the replacement for #298.

[bkoelman]

Suggested change

	You generally don't want callers to be able to change an internal collection, so don't return arrays, lists or other collection classes directly. Instead, return an `IEnumerable<T>`, `IAsyncEnumerable<T>`, `IReadOnlyCollection<T>`, `IReadOnlyList<T>`, `IReadOnlySet<T>` or `IReadOnlyDictionary<TKey, TValue>`.
	You generally don't want callers to be able to change an internal collection, so don't return arrays, lists or other mutable collection classes directly. Instead, return an `IEnumerable<T>`, `IAsyncEnumerable<T>`, `IReadOnlyCollection<T>`, `IReadOnlyList<T>`, `IReadOnlySet<T>` or `IReadOnlyDictionary<TKey, TValue>`.

[bkoelman]

Suggested change

	Be aware that `IEnumerable<T>` is often perceived as lazy-evaluated. If your collection is already materialized, consider returning `IReadOnlyCollection<T>` or even a plain array to make the intent clear.
	Be aware that `IEnumerable<T>` is often perceived as lazy-evaluated. If your collection is already materialized, consider returning `IReadOnlyCollection<T>` or `IReadOnlyList<T>` to make the intent clear.

Returning an array goes against the rule itself: it is mutable (read the first line).

[bkoelman]

Suggested change

	**Exception:** Immutable collections such as `ImmutableArray<T>`, `ImmutableList<T>` and `ImmutableDictionary<TKey, TValue>` prevent modifications from the outside and are thus allowed.
	**Exception:** Immutable collections such as `ImmutableArray<T>`, `ImmutableList<T>` and `ImmutableDictionary<TKey, TValue>`, as well as `FrozenSet<T>` and `FrozenDictionary<TKey, TValue>` prevent modifications from the outside and are thus allowed.