dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source9')

Error message

Value cannot be null. (Parameter 'source9')

What it means

In the larger (11+ source) generated Zip overload, a null ninth sequence throws ArgumentNullException(Parameter 'source9'). All sources and the selector are validated eagerly before Create() builds the operator, so the error is thrown synchronously at the call site.

Solutions

1. Null-check the ninth stream before the Zip call
2. Replace null with AsyncObservable.Empty<T9>() when absence is valid
3. Fix the upstream producer or collection initializer that yielded null
4. Refactor to combine streams incrementally (pairwise Zip) so fewer arguments are positionally error-prone

Example fix

// before
var zipped = AsyncObservable.Zip(s1..s8, GetNinth(), s10, s11, selector); // GetNinth() returned null
// after
var src9 = GetNinth() ?? AsyncObservable.Empty<T9>();
var zipped = AsyncObservable.Zip(s1..s8, src9, s10, s11, selector);

if (source9 is null) source9 = AsyncObservable.Empty<T9>(); // or throw with context

static bool IsValidSource<T>(IAsyncObservable<T>? s) => s is not null;

try
{
    var zipped = AsyncObservable.Zip(s1, s2, s3, s4, s5, s6, s7, s8, source9, s10, s11, selector);
}
catch (ArgumentNullException ex) when (ex.ParamName == "source9")
{
    // fall back to a 10-source composition without source9
}

When it happens

Trigger: Calling the large-arity Zip overload where source9 (IAsyncObservable<T9>) is null.

Common situations: A null element in a programmatically built stream list; an optional ninth feed resolved to null; copy-paste of Zip call left one argument uninitialized.

Related errors

• Value cannot be null. (Parameter 'eighth')
• nameof(removeHandler)
• nameof(target)
• nameof(eventName)
• observer

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/4a8a3c8164ba2f6c. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Zip.Generated.cs:1036

        {
            if (source1 == null)
                throw new ArgumentNullException(nameof(source1));
            if (source2 == null)
                throw new ArgumentNullException(nameof(source2));
            if (source3 == null)
                throw new ArgumentNullException(nameof(source3));
            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 (source8 == null)
                throw new ArgumentNullException(nameof(source8));
            if (source9 == null)
                throw new ArgumentNullException(nameof(source9));
            if (source10 == null)
                throw new ArgumentNullException(nameof(source10));
            if (source11 == null)
                throw new ArgumentNullException(nameof(source11));
            if (selector == null)
                throw new ArgumentNullException(nameof(selector));

            return Create<TResult>(async observer =>
            {
                var d = new CompositeAsyncDisposable();

                var (observer1, observer2, observer3, observer4, observer5, observer6, observer7, observer8, observer9, observer10, observer11) = AsyncObserver.Zip(observer, selector);

                var sub1 = source1.SubscribeSafeAsync(observer1).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub2 = source2.SubscribeSafeAsync(observer2).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub3 = source3.SubscribeSafeAsync(observer3).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub4 = source4.SubscribeSafeAsync(observer4).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub5 = source5.SubscribeSafeAsync(observer5).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();

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