ReactiveX: do something when the first item in the sequence is emitted or the sequence is terminated

RxFinally.cs

using System.Reactive;
using System.Reactive.Linq;
using System.Reactive.Threading.Tasks;

// do something when the first item in the squence is emitted
// or the sequence is terminated

var sequence = Observable
    .Interval(TimeSpan.FromMilliseconds(1000))
    .Do(n => Console.WriteLine(n))
    .Skip(3);

static void Done() => Console.WriteLine("Done");

Console.WriteLine("RunAsync");
{
    using var cts = new CancellationTokenSource(2500);

    using var _ = sequence
        .FirstAsync()
        .Finally(Done)
        .RunAsync(cts.Token);

    Console.ReadLine();
}

Console.WriteLine("ToTask");
{
    using var cts = new CancellationTokenSource(2500);

    _ = sequence
        .FirstAsync()
        .Finally(Done)
        .ToTask(cts.Token);

    Console.ReadLine();
}

Console.WriteLine("TakeUntil, Publish, Connect");
{
    using var cts = new CancellationTokenSource(2500);

    using var _ = sequence
        .FirstAsync()
        .TakeUntil(
            // or: Task.Delay(Timeout.Infinite, cts.Token).ToObservable()
            cts.Token.ToObservable<long>())
        .Finally(Done)
        .Publish()
        .Connect();

    Console.ReadLine();
}

Console.WriteLine("Subscribe via Action<Notification>");
{
    using var cts = new CancellationTokenSource(2500);

    sequence
        .FirstAsync()
        .Finally(Done)
        .Subscribe(
            Observer.ToObserver<long>(n => Console.WriteLine(n)),
            cts.Token);

    Console.ReadLine();
}

Console.WriteLine("Subscribe via onNext/onError/onCompleted");
{
    using var cts = new CancellationTokenSource(2500);

    sequence
        .FirstAsync()
        .Finally(Done)
        .Subscribe(
            onNext: n => Console.WriteLine(n),
            onError: e => Console.WriteLine(e),
            onCompleted: () => Console.WriteLine("completed"), 
            cts.Token);
    
    Console.ReadLine();
}

public static class RxExt
{
    public static IObservable<T> ToObservable<T>(this CancellationToken @this)
    {
        if (@this.IsCancellationRequested)
        {
            return Observable.Throw<T>(new OperationCanceledException(@this));
        }
        if (!@this.CanBeCanceled)
        {
            return Observable.Never<T>();
        }
        return Observable.Create<T>(
            observer => @this.Register(
                (_, token) => observer.OnError(new OperationCanceledException(token)), 
                null));
    }
}