dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source9')

Error message

Value cannot be null. (Parameter 'source9')

What it means

The 9-source Zip overload checks source9 at Observable.Multiple.Zip.cs:413; a null ninth observable throws ArgumentNullException with Parameter 'source9'. Like the other guards, it fires synchronously at the Zip call, giving fail-fast diagnostics before the query ever runs.

Solutions

1. Pass a valid IObservable for source9; substitute Observable.Empty<TSource9>() when the source can legitimately be missing.
2. Repair the producing method so it returns an empty observable instead of null.
3. Add a null-check/normalization step for all sources before composing.
4. Reconsider arity: if sources are frequently optional, compose from a list of sources or use a different combinator that tolerates absence.

Example fix

// before
var z = Observable.Zip(a, b, c, d, e, f, g, h, lastStream, sel); // lastStream == null
// after
var z = Observable.Zip(a, b, c, d, e, f, g, h, lastStream ?? Observable.Empty<T9>(), sel);

if (source9 == null)
    source9 = Observable.Empty<TSource9>();

static bool Bound<T>(IObservable<T>? s) => s is not null;

try
{
    var z = Observable.Zip(source1, source2, source3, source4, source5, source6, source7, source8, source9, selector);
}
catch (ArgumentNullException ex) when (ex.ParamName == "source9")
{
    z = Observable.Empty<TResult>();
}

When it happens

Trigger: Passing null as the ninth (final sequence) IObservable argument of the 9-argument Observable.Zip, e.g. an unassigned field or null returned by a stream factory.

Common situations: Nine-source aggregations where the last feed is newly added and not wired in all environments; partial test stubs; feature-flagged streams disabled leaving a null in the final position.

Related errors

• Value cannot be null. (Parameter 'fifth')
• nameof(onError)
• nameof(onCompleted)
• nameof(witness)
• Value cannot be null. (Parameter 'source9')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/56357e88fdde272c. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Multiple.Zip.cs:413

            if (source6 == null)
            {
                throw new ArgumentNullException(nameof(source6));
            }

            if (source7 == null)
            {
                throw new ArgumentNullException(nameof(source7));
            }

            if (source8 == null)
            {
                throw new ArgumentNullException(nameof(source8));
            }

            if (source9 == null)
            {
                throw new ArgumentNullException(nameof(source9));
            }

            if (resultSelector == null)
            {
                throw new ArgumentNullException(nameof(resultSelector));
            }

            return s_impl.Zip(source1, source2, source3, source4, source5, source6, source7, source8, source9, resultSelector);
        }

        /// <summary>
        /// Merges the specified observable sequences into one observable sequence by using the selector function whenever all of the observable sequences have produced an element at a corresponding index.
        /// </summary>
        /// <typeparam name="TSource1">The type of the elements in the first source sequence.</typeparam>
        /// <typeparam name="TSource2">The type of the elements in the second source sequence.</typeparam>
        /// <typeparam name="TSource3">The type of the elements in the third source sequence.</typeparam>
        /// <typeparam name="TSource4">The type of the elements in the fourth source sequence.</typeparam>
        /// <typeparam name="TSource5">The type of the elements in the fifth source sequence.</typeparam>

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