In general, you 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>, IReadOnlyCollection<T>, IReadOnlyList<T> or IReadOnlyDictionary<T>.

So

    public IEnumerable<FooBar> GetRecentItems()

is preferred to

    public List<FooBar> GetRecentItems()

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