dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source2')

Error message

Value cannot be null. (Parameter 'source2')

What it means

ArgumentNullException thrown by the generated 7-source Zip operator because source2 (the second IAsyncObservable) is null. The generated Zip overloads validate each source and the selector in order, so a null second source is reported with parameter name 'source2'.

Solutions

1. Pass a valid second observable; substitute AsyncObservable.Empty<T2>() if 'no elements' is intended
2. Check where source2 is produced/loaded (DI, array index, property init)
3. Verify argument order in the long Zip call — a shifted argument list is a common cause

Example fix

// before
var zipped = s1.Zip(streams[0], streams[1], streams[2], streams[3], streams[4], streams[5], sel); // streams[0] == null
// after
var s2 = streams[0] ?? AsyncObservable.Empty<T2>();
var zipped = s1.Zip(s2, streams[1], streams[2], streams[3], streams[4], streams[5], sel);

if (source2 is null) throw new InvalidOperationException("Zip requires all source observables; source2 was null.");
var sources = new[] { s1, s2, s3, s4, s5, s6, s7 };
if (sources.Any(s => s is null)) throw new InvalidOperationException("All Zip sources must be non-null.");

bool AllNonNull<T>(params IAsyncObservable<T>?[] sources) => sources.All(s => s is not null);

try { var zipped = s1.Zip(s2, s3, s4, s5, s6, s7, selector); }
catch (ArgumentNullException ex) when (ex.ParamName is "source1" or "source2" or "source3" or "source4" or "source5" or "source6" or "source7")
{
    // substitute empty observables for missing sources or surface pipeline misconfiguration
    throw new InvalidOperationException($"Zip input missing: {ex.ParamName}", ex);
}

When it happens

Trigger: Calling source1.Zip(s2, s3, s4, s5, s6, s7, selector) where the second argument is null — e.g. an array/field element that was never populated. Note source1 must also be non-null (checked first).

Common situations: Building the list of streams from a collection with missing entries; a nullable observable property not yet initialized; misordered arguments in a long call.

Related errors

• Value cannot be null. (Parameter 'source3')
• scheduler
• observer
• Value cannot be null. (Parameter 'observer')
• Value cannot be null. (Parameter 'onCompletedAsync')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/5853b9dbe89b360d. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Zip.Generated.cs:496

                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();
                var sub6 = source6.SubscribeSafeAsync(observer6).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub7 = source7.SubscribeSafeAsync(observer7).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();

                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);

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