1130.md

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>.

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