dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'fifteenth')

Error message

Value cannot be null. (Parameter 'fifteenth')

What it means

The fifteen-source Zip overload ends its argument validation with a null check on the fifteenth observable sequence, throwing ArgumentNullException at Observable.Multiple.Zip.cs:1811 if it is null. System.Reactive throws eagerly so the composed query is guaranteed to have a valid source in every position.

Solutions

1. Pass a valid IObservable<T> as the fifteenth argument, or substitute Observable.Empty<TFifteenth>().
2. Null-check/coalesce all fifteen source variables before composing.
3. Fix the producer of the fifteenth sequence so it never returns null.
4. Use the IEnumerable-based Zip overload when source count is dynamic.
5. Wrap composition in try/catch (ArgumentNullException) with context about the missing source.

Example fix

// before
var zipped = ZipAll(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14, s15); // s15 == null
// after
var zipped = ZipAll(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14, s15 ?? Observable.Empty<T15>());

if (s15 is null) s15 = Observable.Empty<T15>();

static IObservable<T> OrEmpty<T>(IObservable<T>? s) => s ?? Observable.Empty<T>();

try { zipped = ZipAll(s1, ..., s15); }
catch (ArgumentNullException ex) when (ex.ParamName == "fifteenth") { zipped = Observable.Empty<(T1,...,T15)>(); }

When it happens

Trigger: Calling the public static Observable.Zip overload taking up to fifteen IObservable<T> sources with null in the fifteenth (last) argument position, right before s_impl.Zip(first, ..., fifteenth) is invoked.

Common situations: A trailing optional stream left as null after a code change; building the argument list from a collection that is one element short; copy-paste edits that dropped the final source initializer.

Related errors

• Value cannot be null. (Parameter 'twelfth')
• Value cannot be null. (Parameter 'thirteenth')
• Value cannot be null. (Parameter 'fourteenth')
• Value cannot be null. (Parameter 'source10')
• Value cannot be null. (Parameter 'tenth')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/7c5f3ce8cf1a9ecb. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Multiple.Zip.cs:1811

                throw new ArgumentNullException(nameof(sixth));
            if (seventh == null)
                throw new ArgumentNullException(nameof(seventh));
            if (eighth == null)
                throw new ArgumentNullException(nameof(eighth));
            if (ninth == null)
                throw new ArgumentNullException(nameof(ninth));
            if (tenth == null)
                throw new ArgumentNullException(nameof(tenth));
            if (eleventh == null)
                throw new ArgumentNullException(nameof(eleventh));
            if (twelfth == null)
                throw new ArgumentNullException(nameof(twelfth));
            if (thirteenth == null)
                throw new ArgumentNullException(nameof(thirteenth));
            if (fourteenth == null)
                throw new ArgumentNullException(nameof(fourteenth));
            if (fifteenth == null)
                throw new ArgumentNullException(nameof(fifteenth));

            return s_impl.Zip(first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, tenth, eleventh, twelfth, thirteenth, fourteenth, fifteenth);
        }

        /// <summary>
        /// Merges the specified observable sequences into one observable sequence of tuple values whenever all of the observable sequences have produced an element at a corresponding index.
        /// </summary>
        /// <typeparam name="TFirst">The type of the elements in the first source sequence.</typeparam>
        /// <typeparam name="TSecond">The type of the elements in the second source sequence.</typeparam>
        /// <typeparam name="TThird">The type of the elements in the third source sequence.</typeparam>
        /// <typeparam name="TFourth">The type of the elements in the fourth source sequence.</typeparam>
        /// <typeparam name="TFifth">The type of the elements in the fifth source sequence.</typeparam>
        /// <typeparam name="TSixth">The type of the elements in the sixth source sequence.</typeparam>
        /// <typeparam name="TSeventh">The type of the elements in the seventh source sequence.</typeparam>
        /// <typeparam name="TEighth">The type of the elements in the eighth source sequence.</typeparam>
        /// <typeparam name="TNinth">The type of the elements in the ninth source sequence.</typeparam>
        /// <typeparam name="TTenth">The type of the elements in the tenth source sequence.</typeparam>
        /// <typeparam name="TEleventh">The type of the elements in the eleventh source sequence.</typeparam>

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