using System;

using System.Linq;

using System.Reactive.Linq;

using System.Reactive.Subjects;

using System.Reactive.Threading.Tasks;

using System.Reactive.Concurrency;

using System.Threading.Tasks;

public class Program

{

    public static void Main()

    {

        var source = Enumerable.Range(0, 1000).Select(i => (long)i);

        var subject = new Subject<long>();

        var cntTask = subject.Count().ToTask();

        var sumTask = subject.Sum().ToTask();

        var avgTask = subject.Average().ToTask();

        Observable.Using(() => source.GetEnumerator(), enumerator =>

            Observable.Defer(() => Observable.FromAsync(() => Task.FromResult(enumerator.MoveNext() ? enumerator.Current : -1)))

                .ObserveOn(Scheduler.Default)

                .Repeat()

                .TakeWhile(item => item != -1)

        ).Subscribe(subject);

        Console.WriteLine($"Count: {cntTask.Result:#,0}, Sum: {sumTask.Result:#,0}, Average: {avgTask.Result:#,0.0}");

    }

}

​