dotnet/reactive · error · ArgumentNullException

throw new ArgumentNullException(nameof(second))

Error message

throw new ArgumentNullException(nameof(second))

What it means

In the Qbservable (expression-tree) ForkJoin, the second observable sequence is null; the provider validates it because the IQueryable expression for the parallel last-element join cannot be built with a missing operand. Fix: pass a non-null IQbservable/IObservable for the second argument.

Solutions

1. Supply a non-null IObservable as second
2. Null-check second before calling ForkJoin
3. Replace null with Observable.Empty<TSource2>() if 'no source' is intended

Example fix

// before
first.ForkJoin(null, (a, b) => a + b);
// after
first.ForkJoin(second ?? Observable.Empty<int>().AsQbservable(), (a, b) => a + b);

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

bool IsValidSecond<T>(IObservable<T> o) => o is not null;

try { var q = first.ForkJoin(second, selector); } catch (ArgumentNullException ex) when (ex.ParamName == "second") { /* handle null second */ }

When it happens

Trigger: Calling first.ForkJoin(null, resultSelector) or passing a null observable variable as the second source.

Common situations: A source observable that is null because an optional service or config-driven producer did not create it.

Related errors

• throw new ArgumentNullException(nameof(first))
• throw new ArgumentNullException(nameof(resultSelector))
• throw new ArgumentNullException(nameof(sources))
• Value cannot be null. (Parameter 'fourteenth')
• Value cannot be null. (Parameter 'thirteenth')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/b37e921eb869a7df. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/QbservableEx.Homoicon.cs:1148

        /// <summary>
        /// Runs two observable sequences in parallel and combines their last elements.
        /// </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="TResult">The type of the elements in the result sequence, returned by the selector function.</typeparam>
        /// <param name="first">First observable sequence.</param>
        /// <param name="second">Second observable sequence.</param>
        /// <param name="resultSelector">Result selector function to invoke with the last elements of both sequences.</param>
        /// <returns>An observable sequence with the result of calling the selector function with the last elements of both input sequences.</returns>
        /// <exception cref="ArgumentNullException">
        /// <paramref name="first" /> or <paramref name="second" /> or <paramref name="resultSelector" /> is null.</exception>
        [Experimental]
        public static IQbservable<TResult> ForkJoin<TSource1, TSource2, TResult>(this IQbservable<TSource1> first, IObservable<TSource2> second, Expression<Func<TSource1, TSource2, TResult>> resultSelector)
        {
            if (first == null)
                throw new ArgumentNullException(nameof(first));
            if (second == null)
                throw new ArgumentNullException(nameof(second));
            if (resultSelector == null)
                throw new ArgumentNullException(nameof(resultSelector));

            return first.Provider.CreateQuery<TResult>(
                Expression.Call(
                    null,
                    new Func<IQbservable<TSource1>, IObservable<TSource2>, Expression<Func<TSource1, TSource2, TResult>>, IQbservable<TResult>>(ForkJoin<TSource1, TSource2, TResult>).Method,
                    first.Expression,
                    GetSourceExpression(second),
                    resultSelector
                )
            );
        }
#endif

#if !STABLE
        /// <summary>
        /// Returns an observable sequence that is the result of invoking the selector on the source sequence, without sharing subscriptions.

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