dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source13')

Error message

Value cannot be null. (Parameter 'source13')

What it means

The 13-source Observable.CombineLatest overload null-checks all 13 sources and then the resultSelector; this error means the thirteenth source (source13) was null. The last source is validated before the selector so the null-check order matches parameter order.

Solutions

1. Ensure the thirteenth argument is a non-null IObservable<TSource13> before the call.
2. Substitute Observable.Empty<TSource13>() if the final signal is optional (CombineLatest then completes when others finish).
3. Hold Subjects/streams in non-null fields initialized at construction instead of null-then-assign.

Example fix

// before
Observable.CombineLatest(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, trigger, sel); // trigger null
// after
var s13 = trigger ?? Observable.Empty<T13>();
Observable.CombineLatest(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, sel);

if (source13 == null) throw new InvalidOperationException("source13 must be assigned before CombineLatest");

bool IsValidSource13<T13>(IObservable<T13> s) => s is not null;

try { combined = Observable.CombineLatest(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, sel); }
catch (ArgumentNullException ex) when (ex.ParamName == "source13") { log.LogError("source13 was null"); combined = Observable.Empty<TResult>(); }

When it happens

Trigger: Calling Observable.CombineLatest with 13 sources where the final source argument is null, triggering the guard at Observable.Multiple.CombineLatest.cs:814.

Common situations: The last stream (often a completion/trigger signal) never assigned; copy-paste of the call missing the final source; a nullable subject that was disposed and set to null.

Related errors

• Value cannot be null. (Parameter 'source6')
• Value cannot be null. (Parameter 'source7')
• Value cannot be null. (Parameter 'source15')
• Value cannot be null. (Parameter 'twelfth')
• Value cannot be null. (Parameter 'sixteenth')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/6611b52ec518d90a. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Multiple.CombineLatest.cs:814

            if (source10 == null)
            {
                throw new ArgumentNullException(nameof(source10));
            }

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

            return s_impl.CombineLatest(source1, source2, source3, source4, source5, source6, source7, source8, source9, source10, source11, source12, source13, resultSelector);
        }

        /// <summary>
        /// Merges the specified observable sequences into one observable sequence by using the selector function whenever any of the observable sequences produces an element.
        /// </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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