gistfile1.cs

IObservable<SomeResult> CalculateSomeStuff(SomeStuff anItem)
{
}


// Do work on 4 threads at a time
someListOfStuff.ToObservable()
    .Select(x => Observable.Defer(() => CalculateSomeStuff(x)))
    .Merge(4)
    .ToList()
    .Subscribe(x => {
        Console.WriteLine("Sweet I've got a new List");
    });