dotnet/reactive · error · ArgumentNullException

source10

Error message

source10

What it means

System.ArgumentNullException thrown by the public Zip overload combining 14 observable sources in Observable.Multiple.Zip.cs when the tenth source sequence (source10) is null. Rx.NET's Zip operator requires every input sequence to be a live, non-null IObservable<TSource> because it must subscribe to each one to pair elements across sequences. Passing null violates the library's eager null-check contract, so it fails fast at call time instead of inside the subscription pipeline.

Solutions

1. Ensure the tenth argument passed to Zip is a real observable; construct or await it before the call.
2. If the stream is intentionally empty, pass Observable.Empty<T>() instead of null.
3. Guard with ArgumentNullException.ThrowIfNull(s10) / a null check before invoking Zip so the failure surfaces at your own call site with a clearer message.
4. If sources come from a collection, replace the 14-argument overload with Observable.Zip(sources, resultSelector) over a non-empty, null-free list.

Example fix

// before
var zipped = Observable.Zip(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14, (a, b, c, d, e, f, g, h, i, j, k, l, m, n) => j);
// after
ArgumentNullException.ThrowIfNull(s10);
var zipped = Observable.Zip(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10 ?? Observable.Empty<TimeSpan>(), s11, s12, s13, s14, (a, b, c, d, e, f, g, h, i, j, k, l, m, n) => j);

if (s10 is null) throw new ArgumentNullException(nameof(s10));

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

try { var zipped = Observable.Zip(s1, ..., s10, ..., s14, selector); } catch (ArgumentNullException ex) when (ex.ParamName == "source10") { // substitute Observable.Empty<T>() and retry }

When it happens

Trigger: Calling the 14-source Observable.Zip overload, e.g. Observable.Zip(s1, s2, ..., s10, ..., s14, resultSelector), with s10 (the tenth argument) being a null IObservable reference while the other arguments are non-null observables and resultSelector is non-null.

Common situations: Building a fixed-size array of streams where one slot was never initialized (e.g. sources[9] left null after conditional population); refactoring code that replaced a placeholder observable with null instead of Observable.Empty<T>(); DI or factory methods returning null for one of several merged event streams; copying a call site and forgetting to update one of the positional arguments.

Related errors

• source11
• source12
• source13
• source14
• Value cannot be null. (Parameter 'source15')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/0315b91c7fae3e1a. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Multiple.Zip.cs:898

            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 (source12 == null)
            {
                throw new ArgumentNullException(nameof(source12));
            }

            if (source13 == null)
            {
                throw new ArgumentNullException(nameof(source13));
            }

            if (source14 == null)

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