How to use async countdown event instead of collecting tasks and awaiting on them?

Question

I have the following code:

var tasks = await taskSeedSource
    .Select(taskSeed => GetPendingOrRunningTask(taskSeed, createTask, onFailed, onSuccess, sem))
    .ToList()
    .ToTask();

if (tasks.Count == 0)
{
    return;
}

if (tasks.Contains(null))
{
    tasks = tasks.Where(t => t != null).ToArray();
    if (tasks.Count == 0)
    {
        return;
    }
}

await Task.WhenAll(tasks);

Where taskSeedSource is a Reactive Observable. It could be that this code have many problems, but I see at least two:

1. I am collecting tasks whereas I could do without it.
2. Somehow, the returned tasks list may contain nulls, even though GetPendingOrRunningTask is an async method and hence never returns null. I failed to understand why it happens, so I had to defend against it without understanding the cause of the problem.

I would like to use the AsyncCountdownEvent from the AsyncEx framework instead of collecting the tasks and then awaiting on them.

So, I can pass the countdown event to GetPendingOrRunningTask which will increment it immediately and signal before returning after awaiting for the completion of its internal logic. However, I do not understand how to integrate the countdown event into the monad (that is the Reactive jargon, isn't it?).

What is the right way to do it?

EDIT

Guys, let us forget about the mysterious nulls in the returned list. Suppose everything is green and the code is

var tasks = await taskSeedSource
    .Select(taskSeed => GetPendingOrRunningTask(taskSeed, ...))
    .ToList()
    .ToTask();

await Task.WhenAll(tasks);

Now the question is how do I do it with the countdown event? So, suppose I have:

var c = new AsyncCountdownEvent(1);

and

async Task GetPendingOrRunningTask<T>(AsyncCountdownEvent c, T taskSeed, ...)
{
  c.AddCount();
  try
  {
    await ....
  }
  catch (Exception exc)
  {
    // The exception is handled
  }
  c.Signal();  
}

My problem is that I no longer need the returned task. These tasks where collected and awaited to get the moment when all the work items are over, but now the countdown event can be used to indicate when the work is over.

My problem is that I am not sure how to integrate it into the Reactive chain. Essentially, the GetPendingOrRunningTask can be async void. And here I am stuck.

EDIT 2

Strange appearance of a null entry in the list of tasks

Answer 1

@Servy is correct that you need to solve the null Task problem at the source. Nobody wants to answer a question about how to workaround a problem that violates the contracts of a method that you've defined yourself and yet haven't provided the source for examination.

As for the issue about collecting tasks, it's easy to avoid with Merge if your method returns a generic Task<T>:

await taskSeedSource
  .Select(taskSeed => GetPendingOrRunningTask(taskSeed, createTask, onFailed, onSuccess, sem))
  .Where(task => task != null)  // According to you, this shouldn't be necessary.
  .Merge();

However, unfortunately there's no official Merge overload for the non-generic Task but that's easy enough to define:

public static IObservable<Unit> Merge(this IObservable<Task> sources)
{
  return sources.Select(async source =>
  {
    await source.ConfigureAwait(false);
    return Unit.Default;
  })
  .Merge();
}