dotnet/reactive · error · ArgumentNullException

fifteenth

Error message

fifteenth

What it means

The 16-argument QbservableEx.Zip throws ArgumentNullException because the parameter 'fifteenth' (the fifteenth source observable sequence) was null. All sequence arguments are validated up front before an expression tree is created via the IQbservableProvider.

Solutions

1. Assign a real or empty observable to the fifteenth source before calling Zip
2. Add null checks for all 16 arguments prior to the call
3. Fix the factory/dependency that produced the null sequence

Example fix

// before
var q = first.Zip(s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14, s15, s16); // s15 null
// after
var s15 = GetFifteenth() ?? Observable.Empty<TFifteenth>();
var q = first.Zip(s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14, s15, s16);

if (fifteenth == null) throw new ArgumentException(nameof(fifteenth));

static bool IsSource<T>(IObservable<T> s) => s != null;

try { var q = first.Zip(...); } catch (ArgumentNullException ex) { Console.Error.WriteLine($"Missing Zip source: {ex.ParamName}"); }

When it happens

Trigger: Calling the 16-way Zip overload with the fifteenth argument (an IObservable<TFifteenth>) set to null.

Common situations: One of many merged streams comes from a nullable API result or an unresolved dependency in a reactive UI pipeline; copy-paste of the Zip call missed assigning one of the sources.

Related errors

• fourteenth
• sixteenth
• Value cannot be null. (Parameter 'fourteenth')
• source15
• throw new ArgumentNullException(nameof(first))

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/dfe574f953172816. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/QbservableEx.Homoicon.cs:2200

                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));
            if (sixteenth == null)
                throw new ArgumentNullException(nameof(sixteenth));

            return first.Provider.CreateQuery<(TFirst First, TSecond Second, TThird Third, TFourth Fourth, TFifth Fifth, TSixth Sixth, TSeventh Seventh, TEighth Eighth, TNinth Ninth, TTenth Tenth, TEleventh Eleventh, TTwelfth Twelfth, TThirteenth Thirteenth, TFourteenth Fourteenth, TFifteenth Fifteenth, TSixteenth Sixteenth)>(
                Expression.Call(
                    null,
                    new Func<IQbservable<TFirst>, IObservable<TSecond>, IObservable<TThird>, IObservable<TFourth>, IObservable<TFifth>, IObservable<TSixth>, IObservable<TSeventh>, IObservable<TEighth>, IObservable<TNinth>, IObservable<TTenth>, IObservable<TEleventh>, IObservable<TTwelfth>, IObservable<TThirteenth>, IObservable<TFourteenth>, IObservable<TFifteenth>, IObservable<TSixteenth>, IQbservable<(TFirst First, TSecond Second, TThird Third, TFourth Fourth, TFifth Fifth, TSixth Sixth, TSeventh Seventh, TEighth Eighth, TNinth Ninth, TTenth Tenth, TEleventh Eleventh, TTwelfth Twelfth, TThirteenth Thirteenth, TFourteenth Fourteenth, TFifteenth Fifteenth, TSixteenth Sixteenth)>>(Zip<TFirst, TSecond, TThird, TFourth, TFifth, TSixth, TSeventh, TEighth, TNinth, TTenth, TEleventh, TTwelfth, TThirteenth, TFourteenth, TFifteenth, TSixteenth>).Method,
                    first.Expression,
                    GetSourceExpression(second),
                    GetSourceExpression(third),
                    GetSourceExpression(fourth),
                    GetSourceExpression(fifth),
                    GetSourceExpression(sixth),
                    GetSourceExpression(seventh),
                    GetSourceExpression(eighth),
                    GetSourceExpression(ninth),
                    GetSourceExpression(tenth),
                    GetSourceExpression(eleventh),

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