public static ChannelReader<T> OfType<T, S>(this ChannelReader<S> reader, 
                                                 ChannelParallelOptions options)

using Amphisbaena;
using Amphisbaena.Linq;

...

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

var longs = data
  .ToChannelReader()
  .OfType<int, long>();