dotnet/reactive · error · ArgumentNullException

source14

Error message

source14

What it means

System.ArgumentNullException thrown by the 14-source public Zip overload in Observable.Multiple.Zip.cs when the last source sequence (source14) is null. Zip validates each of its fourteen source parameters and finally its resultSelector; a null source14 fails that check and the operator never gets created. The eager throw prevents a null reference deeper inside the zipper when it tried to subscribe to the fourteenth sequence.

Solutions

1. Ensure source14 is assigned to a valid observable before calling Zip.
2. If the last stream is optional, pass Observable.Empty<T>() (note: Zip yields no items if any input is empty, so consider CombineLatest or skipping that source).
3. Guard with ArgumentNullException.ThrowIfNull(s14) prior to the call for a clearer local failure.
4. Consolidate sources into an array and use the collection Zip overload, asserting all elements are non-null beforehand.

Example fix

// before
var zipped = Observable.Zip(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14, selector); // s14 == null
// after
var zipped = Observable.Zip(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14 ?? Observable.Empty<bool>(), selector);

if (s14 is null) throw new ArgumentNullException(nameof(s14));

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

try { var zipped = Observable.Zip(s1, ..., s13, s14, selector); } catch (ArgumentNullException ex) when (ex.ParamName == "source14") { // use Observable.Empty<T>() or restructure with CombineLatest }

When it happens

Trigger: Calling Observable.Zip(s1, ..., s13, s14, resultSelector) with the fourteenth argument as a null IObservable reference (resultSelector itself non-null, since that check follows).

Common situations: A chain of streams extended one item too far where the last stream was never wired (e.g. a background producer that failed to start and left its Subject/observable field null); copy-paste of a Zip call from a 13-source snippet with one extra argument added; null returned by a service that resolves the final stream.

Related errors

• source10
• source11
• source12
• source13
• Value cannot be null. (Parameter 'source15')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/2fcd4f3f073f058a. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Multiple.Zip.cs:918

            if (source11 == null)
            {
                throw new ArgumentNullException(nameof(source11));
            }

            if (source12 == null)
            {
                throw new ArgumentNullException(nameof(source12));
            }

            if (source13 == null)
            {
                throw new ArgumentNullException(nameof(source13));
            }

            if (source14 == null)
            {
                throw new ArgumentNullException(nameof(source14));
            }

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

            return s_impl.Zip(source1, source2, source3, source4, source5, source6, source7, source8, source9, source10, source11, source12, source13, source14, 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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