public static async Task<R> Max<T, R>(this ChannelReader<T> reader,
                                           Func<T, R> selector,
                                           IComparer<R> comparer,
                                           ChannelParallelOptions options)

using Amphisbaena;
using Amphisbaena.Linq;

...

int[] data = new int[] {1, 2, 3};

int max = await data
  .ToChannelReader()
  .Max(item => item);