Step1.cs

using System.Security.Cryptography;
using System.Threading.Tasks.Dataflow;

// The recursive block: takes a folder path, returns all entries inside it.
// TransformManyBlock is ideal here because one folder produces many entries.
var getFolderContents = new TransformManyBlock<string, string>(folder =>
{
    return Directory.GetFileSystemEntries(folder);
});

// Computes the MD5 hash of a file and pairs it with the filename.
var computeMD5 = new TransformBlock<string, (string FilePath, string Hash)>(filePath =>
{
    using var md5 = MD5.Create();
    using var stream = File.OpenRead(filePath);
    var hashBytes = md5.ComputeHash(stream);
    var hash = BitConverter.ToString(hashBytes).Replace("-", "");

    return (filePath, hash);
});

// Displays the result.
var displayResult = new ActionBlock<(string FilePath, string Hash)>(result =>
{
    Console.WriteLine($"{result.Hash}  {Path.GetFileName(result.FilePath)}");
});