dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source14')

Error message

Value cannot be null. (Parameter 'source14')

What it means

The CombineLatest overload combining 14 source sequences validates each argument eagerly. When the 14th IObservable<TSource14> argument is null, it throws ArgumentNullException naming 'source14'. Rx's convention is synchronous fail-fast: the exception surfaces at the CombineLatest call itself, not during subscription.

Solutions

1. Ensure the value passed as source14 is a real, non-null IObservable<TSource14>.
2. Use Observable.Empty<TSource14>() or a replayed/subject-based default for an optional final stream instead of null.
3. Validate all sources before the call (e.g. a guarded factory method that throws with a clearer message).

Example fix

// before
var combined = Observable.CombineLatest(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, finalStream, sel);
// after
var combined = Observable.CombineLatest(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, finalStream ?? Observable.Empty<Stream14>(), sel);

if (source14 == null) source14 = Observable.Empty<Stream14>();
var combined = Observable.CombineLatest(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, source14, selector);

bool HasStream<T>(IObservable<T> s) => s is not null;

try
{
    var combined = Observable.CombineLatest(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, source14, selector);
}
catch (ArgumentNullException ex) when (ex.ParamName == "source14")
{
    logger.LogError(ex, "14th CombineLatest source was null");
}

When it happens

Trigger: Calling Observable.CombineLatest with 14 sources and passing null as the last source (source14), e.g. from an unassigned variable, a nullable field, or a factory that returned null for the final stream.

Common situations: The last argument is easy to miss when a long argument list is edited; adding a 15th argument or trimming an overload call and misplacing a null; a conditional stream that defaulted to null instead of Observable.Empty.

Related errors

• Value cannot be null. (Parameter 'source10')
• Value cannot be null. (Parameter 'source')
• Value cannot be null. (Parameter 'scheduler')
• Value cannot be null. (Parameter 'observer')
• comparer

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/57b98eca2249967b. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Multiple.CombineLatest.cs:930

            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.CombineLatest(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 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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