ObservableExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reactive.Linq;
using System.ComponentModel;

namespace CosmicShores.Core.Rx {
    public static class ObservableExtensions {
        // PERF can be improved
        // TODO propose integration this into System.Reactive itself. (https://github.com/dotnet/reactive/issues/1791#issuecomment-1186218797)
        public static IObservable<IList<TSource>> CombineLatest<TSource>(this IEnumerable<IObservable<TSource>> sources, SourcelessBehaviour noSourceBehaviour) {
            switch (noSourceBehaviour) {
                case SourcelessBehaviour.SingleEmptyArray:
                    if (sources is null)
                        throw new ArgumentNullException(nameof(sources));

                    // wrapping within Observable.Create so the IEnumerable isn't executed earlier than it normally would be.
                    return Observable.Create<IList<TSource>>(SingleEmptyArrayBehaviourSubscribe);

                case SourcelessBehaviour.Default:
                    return sources.CombineLatest();

                default:
                    throw new InvalidEnumArgumentException(nameof(noSourceBehaviour), (int)noSourceBehaviour, typeof(SourcelessBehaviour));
            }

            IDisposable SingleEmptyArrayBehaviourSubscribe(IObserver<IList<TSource>> observer) {
                var sourceArray = sources.ToArray();
                var observable = sourceArray.Length == 0
                    ? Observable.Return(Array.Empty<TSource>())
                    : sourceArray.CombineLatest();
                return observable.Subscribe(observer);
            }
        }

        // PERF can be improved
        public static IObservable<TResult> CombineLatest<TSource, TResult>(this IEnumerable<IObservable<TSource>> sources, Func<IList<TSource>, TResult> resultSelector, SourcelessBehaviour noSourceBehaviour) {
            if (resultSelector is null) throw new ArgumentNullException(nameof(resultSelector));
            return sources.CombineLatest(noSourceBehaviour).Select(resultSelector);
        }
    }
}

SourcelessBehaviour.cs

namespace CosmicShores.Core.Rx {
    /// <summary>
    /// Behaviour for emiting when there are no sources.
    /// </summary>
    public enum SourcelessBehaviour : byte {
        /// <summary>
        /// Never emits (as of System.Reactive 5.0)
        /// </summary>
        Default = 0,
        SingleEmptyArray = 1,
    }
}