What causes "Value cannot be null. (Parameter 'source8')" in dotnet/reactive?

Provide a non-null eighth observable, using Observable.Empty () or Observable.Never () when absence is legitimate.

dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source8')

Error message

Value cannot be null. (Parameter 'source8')

What it means

The 8-source CombineLatest overload validates all eight sources plus resultSelector, throwing ArgumentNullException for the first null. Here source8 was null, so the guard at line 339 fired. Note the final overload is checked before resultSelector, so a null selector would be reported separately.

Solutions

Provide a non-null eighth observable, using Observable.Empty<T8>() or Observable.Never<T8>() when absence is legitimate.
Add a null check on source8 at the call site.
Fix the code path that leaves source8 unassigned.
After adding/removing sources, recheck the full argument list against the parameter names.

Example fix

// before
var combined = Observable.CombineLatest(s1, s2, s3, s4, s5, s6, s7, s8, sel); // s8 == null
// after
var combined = Observable.CombineLatest(s1, s2, s3, s4, s5, s6, s7, s8 ?? Observable.Never<T8>(), sel);

Defensive patterns

Strategy: validation

Validate before calling

if (source8 == null) source8 = Observable.Never<TSource8>();
if (resultSelector == null) throw new ArgumentNullException(nameof(resultSelector));

Type guard

static bool Complete(IObservable<T8> s8, Func<...> sel) => s8 is not null && sel is not null;

Try / catch

try
{
    var combined = Observable.CombineLatest(s1, s2, s3, s4, s5, s6, s7, s8, sel);
}
catch (ArgumentNullException ex) when (ex.ParamName == "source8")
{
    // supply the missing eighth stream, then recompose
}

Prevention

When adding sources to a composition, recheck every slot including the last
Never return null from wrapper methods that promise IObservable<T>
Default uninitialized streams to Observable.Never in constructors
Run nullable reference type analysis to catch unset observables

When it happens

Trigger: Calling Observable.CombineLatest with null in the eighth source slot — e.g. the last stream of a fixed-width composition that was conditionally created.

Common situations: Last observable in a generated/pipeline-composed set not initialized; null returned by a wrapper method for the final stream; positional-argument mix-ups when adding the eighth source during refactoring.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/c932c5fb39e0eee1. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Multiple.CombineLatest.cs:339

            if (source5 == null)
            {
                throw new ArgumentNullException(nameof(source5));
            }

            if (source6 == null)
            {
                throw new ArgumentNullException(nameof(source6));
            }

            if (source7 == null)
            {
                throw new ArgumentNullException(nameof(source7));
            }

            if (source8 == null)
            {
                throw new ArgumentNullException(nameof(source8));
            }

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

            return s_impl.CombineLatest(source1, source2, source3, source4, source5, source6, source7, source8, 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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