dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source5')

Error message

Value cannot be null. (Parameter 'source5')

What it means

The same 7-source Zip overload validates its arguments in order and throws ArgumentNullException when the 5th source observable is null. Zip needs every participating sequence, so a null in any slot aborts operator construction before subscription.

Solutions

1. Supply a non-null IAsyncObservable for source5, or use AsyncObservable.Empty<T5>() when it can legitimately be absent.
2. Add an up-front null check or require-non-null helper for each source before invoking Zip.
3. Refactor to a lower-arity Zip overload or a Merge-based composition if one input is truly optional.

Example fix

// before
var zipped = AsyncObservable.Zip(a, b, c, d, fifthOrNull, f, g);
// after
if (fifthOrNull == null) throw new InvalidOperationException("fifth source missing");
var zipped = AsyncObservable.Zip(a, b, c, d, fifthOrNull, f, g);

var sources = new[] { s1, s2, s3, s4, s5, s6, s7 };
if (sources.Any(s => s == null)) throw new InvalidOperationException("all seven zip sources must be non-null");

bool IsValidZipInput<T>(IAsyncObservable<T> s) => s is not null;

try
{
    var zipped = AsyncObservable.Zip(s1, s2, s3, s4, s5, s6, s7);
}
catch (ArgumentNullException ex) when (ex.ParamName == "source5")
{
    logger.LogError(ex, "5th zip source was null");
}

When it happens

Trigger: Calling AsyncObservable.Zip(source1, source2, source3, source4, null, source6, source7) with a null fifth IAsyncObservable argument.

Common situations: The fifth stream comes from a nullable return value (e.g. a cache miss returning null instead of an empty observable); list/dictionary-driven wiring where one entry is missing; copy-pasted zip calls after a source was renamed and left uninitialized.

Related errors

• Value cannot be null. (Parameter 'source4')
• Value cannot be null. (Parameter 'source6')
• Value cannot be null. (Parameter 'source7')
• Value cannot be null. (Parameter 'selector')
• Value cannot be null. (Parameter 'source1')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/a54f5530b90bd3e1. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Zip.Generated.cs:502

                await Task.WhenAll(sub1, sub2, sub3, sub4, sub5, sub6, sub7).ConfigureAwait(false);

                return d;
            });
        }

        public static IAsyncObservable<TResult> Zip<T1, T2, T3, T4, T5, T6, T7, TResult>(this IAsyncObservable<T1> source1, IAsyncObservable<T2> source2, IAsyncObservable<T3> source3, IAsyncObservable<T4> source4, IAsyncObservable<T5> source5, IAsyncObservable<T6> source6, IAsyncObservable<T7> source7, Func<T1, T2, T3, T4, T5, T6, T7, TResult> selector)
        {
            if (source1 == null)
                throw new ArgumentNullException(nameof(source1));
            if (source2 == null)
                throw new ArgumentNullException(nameof(source2));
            if (source3 == null)
                throw new ArgumentNullException(nameof(source3));
            if (source4 == null)
                throw new ArgumentNullException(nameof(source4));
            if (source5 == null)
                throw new ArgumentNullException(nameof(source5));
            if (source6 == null)
                throw new ArgumentNullException(nameof(source6));
            if (source7 == null)
                throw new ArgumentNullException(nameof(source7));
            if (selector == null)
                throw new ArgumentNullException(nameof(selector));

            return Create<TResult>(async observer =>
            {
                var d = new CompositeAsyncDisposable();

                var (observer1, observer2, observer3, observer4, observer5, observer6, observer7) = AsyncObserver.Zip(observer, selector);

                var sub1 = source1.SubscribeSafeAsync(observer1).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub2 = source2.SubscribeSafeAsync(observer2).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub3 = source3.SubscribeSafeAsync(observer3).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub4 = source4.SubscribeSafeAsync(observer4).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub5 = source5.SubscribeSafeAsync(observer5).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();

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