public static async Task<T[]> ToListAsync<T>(this ChannelReader<T> source, 
                                                   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()
  .ToListAsync();