dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'twelfth')

Error message

Value cannot be null. (Parameter 'twelfth')

What it means

In the 12-source CombineLatest overload, System.Reactive throws ArgumentNullException when the twelfth (last) observable sequence is null. All arguments are null-checked before s_impl.CombineLatest is invoked, so the throw is synchronous and occurs before any subscription. The parameter name in the message identifies the null argument.

Solutions

1. Provide a valid IObservable<T> as the twelfth argument; fall back to Observable.Empty<T>() if it is intentionally absent.
2. Check the assignment/lookup that produces the twelfth sequence and fix the null path.
3. Migrate to the collection overload CombineLatest(sources, resultSelector) and validate the list length/nulls once.
4. Apply a null-coalescing default at the call site.

Example fix

// before
var combined = Observable.CombineLatest(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, null); // twelfth null
// after
var twelfth = GetTwelfth() ?? Observable.Empty<int>();
var combined = Observable.CombineLatest(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, twelfth);

if (twelfth is null) throw new InvalidOperationException("twelfth source must be provided");
twelfth ??= Observable.Empty<TTwelfth>();

static bool IsNotNull<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);
}
catch (ArgumentNullException ex) when (ex.ParamName == "twelfth")
{
    logger.LogWarning("twelfth source was null");
    throw;
}

When it happens

Trigger: Calling Observable.CombineLatest(first..twelfth) with null as the twelfth (Twelfth) IObservable<T> argument, e.g. omitting the last source when copying an example or passing a nullable stream variable that was never assigned.

Common situations: Copy-paste edits of long argument lists dropping the final argument; optional last stream (e.g. telemetry) that is null in some environments; deserialized configuration where the 12th stream binding is missing.

Related errors

• Value cannot be null. (Parameter 'source6')
• Value cannot be null. (Parameter 'source7')
• Value cannot be null. (Parameter 'source13')
• Value cannot be null. (Parameter 'source15')
• Value cannot be null. (Parameter 'sixteenth')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/b2bb6a5ee7716057. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Multiple.CombineLatest.cs:1624

                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));

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

        /// <summary>
        /// Merges the specified observable sequences into one observable sequence of tuple values whenever any of the observable sequences produces an element.
        /// </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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