Reuse result from first observable in second in c#

Result of first observable (itemsCache.GetAllKeys()) is list of strings. If that list contains certain string (DOKUMENTI_KEY), then second observable should be caled. If first observable doesent contain certain string (DOKUMENTI_KEY), then empty list should be returned.

public static IObservable<TResult> If<TSource, TResult>(
            this IObservable<TSource> source,
            Func<TSource, bool> predicate,
            Func<TSource, IObservable<TResult>> thenSource,
            Func<TSource, IObservable<TResult>> elseSource)
        {
            return source
              .SelectMany(
                value => predicate(value)
                  ? thenSource(value)
                  : elseSource(value));
        }

public IObservable<List<Dokument>> GetDokumenti()
        {
            return Observable.If(
                () => itemsCache.GetAllKeys().SelectMany(x => x).Contains(DOKUMENTI_KEY).GetAwaiter().GetResult(),
                itemsCache.GetAllKeys().SelectMany(y => y).SelectMany(z => itemsCache.GetObject<List<Dokument>>(z)),
                Observable.Return(new List<Dokument>())
                );
        }

Is there a better way to do this?