public static ChannelReader<T> ForEach<S, T>(this ChannelReader<S> reader,
                                                  Func<S, T> action,
                                                  ChannelParallelOptions options)

int[] data = Enumerable
  .Range(0, 1000)
  .ToArray();

// Here we split initial data into 100 items batches: 1, 2, ..., 1000 => {1, 2, ..., 100}, {101 ... 200}, ...
// And then sum items within each blocks in parallel  {1, 2, ..., 100}, {101 ... 200}, => 5050, 15050, .. 
// Finally we aggregate (sum) all partial sums         5050 + 15050 + ... => 500500 
int sum = await data
  .ToChannelReader()
  .ToBatch((batch, item, index) => batch.Count < 100)
  .ForEach(batch => batch.Sum(), new ChannelParallelOptions() { DegreeOfParallelism = 4 })
  .Aggregate((s, a) => s + a);