dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'twelfth')

Error message

Value cannot be null. (Parameter 'twelfth')

What it means

System.Reactive's Zip implementation guards each of its many observable source parameters with ArgumentNullException. A null twelfth sequence hits the check at Observable.Multiple.Zip.cs:1805, throwing with Parameter 'twelfth'. The library throws eagerly so the invalid query fails at construction rather than at subscription time.

Solutions

1. Provide a non-null IObservable<T> for the twelfth source; use Observable.Empty<TTwelfth>() when the source is intentionally absent.
2. Validate or coalesce all source arguments before calling Zip.
3. Fix the initialization/configuration that produced the null twelfth stream.
4. Use the IEnumerable-based Zip overload when the source count varies.
5. Catch ArgumentNullException at the composition site and log which positional source was missing.

Example fix

// before
var merged = Zip15(s1, ..., s12, s13, s14, s15); // s12 == null
// after
var merged = Zip15(s1, ..., s12 ?? Observable.Empty<T12>(), s13, s14, s15);

if (s12 is null) throw new InvalidOperationException("twelfth source not initialized");

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

try { merged = Zip15(s1, ..., s15); }
catch (ArgumentNullException ex) when (ex.ParamName == "twelfth") { merged = Observable.Empty<(T1,...,T15)>(); }

When it happens

Trigger: Calling the public static Observable.Zip overload accepting up to fifteen IObservable<T> sources with null in the twelfth argument position, e.g. Observable.Zip(s1, ..., s11, null, s13, s14, s15).

Common situations: Optional streams in a feature-flagged pipeline left null when a feature is off; arrays of feeds iterated positionally with a missing element; migration from a different reactive library that tolerated null sources.

Related errors

• Value cannot be null. (Parameter 'thirteenth')
• Value cannot be null. (Parameter 'fourteenth')
• Value cannot be null. (Parameter 'fifteenth')
• 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/0e8b9ce35d4606cf. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Multiple.Zip.cs:1805

                throw new ArgumentNullException(nameof(third));
            if (fourth == null)
                throw new ArgumentNullException(nameof(fourth));
            if (fifth == null)
                throw new ArgumentNullException(nameof(fifth));
            if (sixth == null)
                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>

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