ReactiveExtensionsExtensions.cs

    public static class ReactiveExtensionsExtensions
    {
        public static async Task SubscribeAsync<T>(this IObservable<T> observable, Action<T> onNext, CancellationToken? cancellationToken = null)
        {
            CancellationToken token = cancellationToken ?? CancellationToken.None;
            var cts = new TaskCompletionSource<bool>();
            using(token.Register(() => cts.SetCanceled()))
            {
                observable.Subscribe(onNext, ex => cts.SetException(ex), () => cts.SetResult(true), token);
                await cts.Task;
            }
        }
    }