LimitConcurrencyLevelScheduler.cs

public class LimitConcurrencyLevelScheduler : LocalScheduler
{
  private readonly ActionBlock<Action> _actionBlock;

  public LimitConcurrencyLevelScheduler(int concurrencyLevel = 0, bool singleProducerConstrained = false)
  {
    var options = new ExecutionDataflowBlockOptions
    {
      MaxDegreeOfParallelism = concurrencyLevel <= 0 ? 1 : concurrencyLevel,
      SingleProducerConstrained = singleProducerConstrained
    };

   _actionBlock = new ActionBlock<Action>(a => a(), options);
  }

  public override IDisposable Schedule<TState>(TState state, TimeSpan dueTime, Func<IScheduler, TState, IDisposable> action)
  {
    if (dueTime.Ticks > 0)
      throw new NotSupportedException();

    var m = new SingleAssignmentDisposable();

    _actionBlock.Post(() =>
    {
      if (!m.IsDisposed)
        m.Disposable = action(this, state);
    });

    return m;
  }
}