dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source10')

Error message

Value cannot be null. (Parameter 'source10')

What it means

System.Reactive's 10-argument Zip overload validates every argument eagerly when the public Zip method is called. Passing a null IObservable for the tenth source sequence causes an ArgumentNullException with Parameter 'source10' to be thrown immediately, before any subscription occurs. This fail-fast contract ensures invalid usage is caught at the call site rather than deep inside a subscription pipeline.

Solutions

1. Ensure the tenth argument passed to Zip is a non-null IObservable; initialize it before the Zip call.
2. If the source may legitimately be absent, substitute Observable.Empty<TSource10>() or a default sequence instead of null.
3. Check where the tenth sequence is produced (a prior Rx operator or factory) and fix whatever returns null - Rx operators themselves never return null.
4. Wrap configuration/resolution of the sources in null checks before composing the query.

Example fix

// before
IObservable<int> source10 = condition ? obsA : null;
var zipped = Observable.Zip(s1, s2, s3, s4, s5, s6, s7, s8, s9, source10, (a,b,c,d,e,f,g,h,i,j) => a);
// after
IObservable<int> source10 = condition ? obsA : Observable.Empty<int>();
var zipped = Observable.Zip(s1, s2, s3, s4, s5, s6, s7, s8, s9, source10, (a,b,c,d,e,f,g,h,i,j) => a);

if (source1 == null) throw new InvalidOperationException("source1 must be provided"); // ...repeat for sources 2-9
if (source10 == null)
{
    source10 = Observable.Empty<TSource10>(); // or fix initialization
}

bool IsValidSource<T>(IObservable<T> s) => s is not null;
// usage: if (sources.All(IsValidSource)) { /* call Zip */ }

try
{
    var zipped = Observable.Zip(s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, selector);
}
catch (ArgumentNullException ex) when (ex.ParamName == "source10")
{
    // log which argument was null and repair initialization
}

When it happens

Trigger: Calling Observable.Zip(source1..source9, source10, resultSelector) (the 10-source overload in Observable.Multiple.Zip.cs) where the tenth IObservable<TSource10> argument is null.

Common situations: Chaining Zip calls where an earlier operator returned null; holding sequences in nullable fields/properties that were never initialized; wiring up sources from a dictionary or service locator where the tenth entry is missing.

Related errors

• ArgumentNullException(nameof(source))
• ArgumentNullException(nameof(other))
• Value cannot be null. (Parameter 'onCompletedAsync')
• Value cannot be null. (Parameter 'onNext')
• Value cannot be null. (Parameter 'onError')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/07a0652c64d3ed3e. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Multiple.Zip.cs:500

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

            return s_impl.Zip(source1, source2, source3, source4, source5, source6, source7, source8, source9, source10, resultSelector);
        }

        /// <summary>
        /// Merges the specified observable sequences into one observable sequence by using the selector function whenever all of the observable sequences have produced an element at a corresponding index.
        /// </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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