public IObservable<IEvent<EventArgs>> PushOnceMethod()
{
    return Observable.Create<IEvent<EventArgs>>(observer => {
        EventHandler<EventArgs> handler = (sender, args) => { observer.OnNext(Event.Create(sender, args)); observer.OnCompleted(); };
        MyEvent += handler;
        return () => MyEvent -= handler;
    });
}

publisher.PushOnceMethod().Subscribe(evt => {...evt.EventArgs...}/*через лямбду*/);

IObservable<IEvent<EventArgs>> observableStoryboard = Observable.FromEvent(
           handler =>  widthStoryboard.Completed += handler, 
           handler =>  widthStoryboard.Completed -= handler)
           .Take(1) //Отписываемся сразу после первого события;

observableStoryboard.Subscribe(args => UpdateSize());

widthStoryboard.Begin();

var obs= 
           from args in  Observable.FromEvent(
           handler =>  widthStoryboard.Completed += handler, 
           handler =>  widthStoryboard.Completed -= handler)
           from e in ....
           let res = new{E =e, Arg = args}
           from c in Observable.Return( new MyClass(...)) 
           from m in c.Method1.ToAsync()
           from res in m(z,res)
           from r in 
           .....
           select res, r

namespace RxText
{
     class Program
    {
        
         public static IEnumerable<IObservable<int>> GetCoroutines(bool condition)
        {
           yield return Observable.Start(() =>
                                              {
                                                  Console.WriteLine   (Thread.CurrentThread.ManagedThreadId.ToString());
                                                  Thread.Sleep(1000);
                                                  return 1;
                                              });
    if (condition)
        yield return Observable.Start(() =>
                                              {
                                                  Console.WriteLine(Thread.CurrentThread.ManagedThreadId.ToString());
                                                  Thread.Sleep(2000);
                                                  return 2;
                                              });
        }

        static void Main(string[] args)
        {
            var ii = from i in GetCoroutines(true).Concat()
                     select  i * 2;
            ii.Sum().Subscribe(r => Console.WriteLine("if true result="+r.ToString()));
      
            var ii1 = from i in GetCoroutines(true).Concat()
                    select i * 2;
            ii1.Sum().Subscribe(r => Console.WriteLine(("if false result="+r.ToString()));
            Console.ReadKey();
        }
    }
}