MyObjectWithALock.cs

class MyObjectWithALock : IDisposable, IAsyncDisposable
{
  private SemaphoreSlim semaphore = new SemaphoreSlim(1, 1);

  public void Dispose()
  {
    semaphore.Dispose();
  }

  public async ValueTask DisposeAsync()
  {
    // Please, no, no.
    await Task.Run(() => semaphore.Dispose());
  }
}