✅ How to avoid blocking until all subscribed handlers complete handling an event

public sealed class MessageBroker
{
  private readonly ConcurrentDictionary<string, byte> m_sentMessages = new();
  private readonly Action<MessageMetadata, string> m_onNewMessage;
  private readonly FileSystemWatcher m_watcher = new();

  public MessageBroker(string exchangePath, Action<MessageMetadata, string> onNewMessage)
  {
    m_onNewMessage = onNewMessage ?? throw new ArgumentNullException(nameof(onNewMessage));

    m_watcher.Path = exchangePath;
    m_watcher.Filter = "[*]_[*]_[*]_[*]_[*]";
    m_watcher.EnableRaisingEvents = true;
    m_watcher.IncludeSubdirectories = false;
    m_watcher.InternalBufferSize = 16_384;
    m_watcher.Created += WatcherOnCreated;
  }

  private static async ValueTask WaitForFileAsync(string filePath)
  {
    // Some Async method
  }

  private async ValueTask ProcessNewMessageAsync(string fullPath)
  {
    if (m_sentMessages.TryRemove(fullPath, out var _))
      return;
    if (!MessageHelper.TryParseToMetadata(fullPath, out var metadata) || metadata is null)
      return;

    await WaitForFileAsync(fullPath).ConfigureAwait(false);

    m_onNewMessage(metadata.Value, fullPath);
  }

  private async void WatcherOnCreated(object sender, FileSystemEventArgs e)
    => await ProcessNewMessageAsync(e.FullPath).ConfigureAwait(false);
}

public sealed class MessageBroker
{
  private readonly ConcurrentDictionary<string, byte> m_sentMessages = new();
  private readonly Action<MessageMetadata, string> m_onNewMessage;
  private readonly FileSystemWatcher m_watcher = new();

  public MessageBroker(string exchangePath, Action<MessageMetadata, string> onNewMessage)
  {
    m_onNewMessage = onNewMessage ?? throw new ArgumentNullException(nameof(onNewMessage));

    m_watcher.Path = exchangePath;
    m_watcher.Filter = "[*]_[*]_[*]_[*]_[*]";
    m_watcher.EnableRaisingEvents = true;
    m_watcher.IncludeSubdirectories = false;
    m_watcher.InternalBufferSize = 16_384;
    m_watcher.Created += WatcherOnCreated;
  }

  private static async ValueTask WaitForFileAsync(string filePath)
  {
    // Some Async method
  }

  private async ValueTask ProcessNewMessageAsync(string fullPath)
  {
    if (m_sentMessages.TryRemove(fullPath, out var _))
      return;
    if (!MessageHelper.TryParseToMetadata(fullPath, out var metadata) || metadata is null)
      return;

    await WaitForFileAsync(fullPath).ConfigureAwait(false);

    m_onNewMessage(metadata.Value, fullPath);
  }

  private async void WatcherOnCreated(object sender, FileSystemEventArgs e)
    => await ProcessNewMessageAsync(e.FullPath).ConfigureAwait(false);
}