What causes "Value cannot be null. (Parameter 'source7')" in dotnet/reactive?

Ensure the seventh argument is a non-null IObservable .

dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source7')

Error message

Value cannot be null. (Parameter 'source7')

What it means

The 7-source CombineLatest overload (Observable.Multiple.CombineLatest.cs:265) throws ArgumentNullException when the seventh source observable is null. This is the last source checked before the resultSelector; CombineLatest subscribes to all seven streams and emits the latest combination whenever any stream produces a value, so every source must be non-null.

Solutions

Ensure the seventh argument is a non-null IObservable<TSource7>.
Default with source7 ?? Observable.Empty<TSource7>().
Move query construction after the point where all streams are guaranteed initialized.

Example fix

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

Defensive patterns

Strategy: validation

Validate before calling

if (source7 is null) source7 = Observable.Empty<T7>();

Type guard

static bool IsSourceValid<T>(IObservable<T>? s) => s is not null;

Try / catch

try { var combined = Observable.CombineLatest(s1, s2, s3, s4, s5, s6, s7, sel); }
catch (ArgumentNullException ex) when (ex.ParamName == "source7") { combined = Observable.CombineLatest(s1, s2, s3, s4, s5, s6, Observable.Empty<T7>(), sel); }

Prevention

Initialize trailing streams (timers, signals) in constructors, not lazily.
Delay query construction until all subjects/streams are created.
Coalesce nullable fields at the call site: source7 ?? Observable.Empty<T7>().
Review refactors that reduce stream count; update overloads, don't pass null.

When it happens

Trigger: Calling the 7-argument Observable.CombineLatest overload with a null seventh observable.

Common situations: The trailing stream (often a timer or shutdown signal) not created in some code paths; nullable readonly fields assigned in Initialize() that the query reads before initialization; conditional pipelines skipping the last stream.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/c8954e488ed1fb93. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Multiple.CombineLatest.cs:265

            if (source4 == null)
            {
                throw new ArgumentNullException(nameof(source4));
            }

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

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