public static async Task<T> LastOrDefault<T>(this ChannelReader<T> reader,
                                                  Func<T, bool> condition,
                                                  T defaultValue,
                                                  ChannelParallelOptions options)

using Amphisbaena;
using Amphisbaena.Linq;

...

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

int last = await data
  .ToChannelReader()
  .LastOrDefault();