public static async Task<HashSet<T>> ToHashSet<T>(this ChannelReader<T> reader,
                                                       IEqualityComparer<T> comparer,
                                                       ChannelParallelOptions options)

using Amphisbaena;
using Amphisbaena.Linq;

...

int[] data = new int[] {1, 2, 3, 4, 5, 6, 7, 8, 9};

var items = await data
  .ToChannelReader()
  .ToHashSetAsync();