dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'onNext')

Error message

Value cannot be null. (Parameter 'onNext')

What it means

Thrown by the notification-based SelectMany(source, onNext, onError, onCompleted) overload when the onNext delegate (Func<TSource, IObservable<TResult>>) is null. Rx requires all three notification handlers because each notification kind must map to a sequence; a null onNext is rejected eagerly at call time.

Solutions

1. Provide an onNext lambda projecting each element to an IObservable<TResult>, e.g. x => Observable.Return(Process(x)).
2. If elements should be ignored, pass x => Observable.Empty<TResult>() instead of null.
3. Use a different SelectMany overload (single selector) if no per-notification handling is required.

Example fix

// before
var q = source.SelectMany(null, err => Observable.Throw<Item>(err), () => Observable.Empty<Item>());

// after
var q = source.SelectMany(x => Observable.Return(Process(x)), err => Observable.Throw<Item>(err), () => Observable.Empty<Item>());

if (onNext is null) throw new ArgumentNullException(nameof(onNext)); // check before invoking SelectMany

static bool HandlersValid<TSource, TResult>(Func<TSource, IObservable<TResult>> onNext)
    => onNext is not null;

try
{
    var q = source.SelectMany(onNext, onError, onCompleted);
}
catch (ArgumentNullException ex) when (ex.ParamName == "onNext")
{
    // substitute a default element mapping
    q = source.SelectMany(x => Observable.Empty<TResult>(), onError, onCompleted);
}

When it happens

Trigger: Calling Observable.SelectMany(source, null, onError, onCompleted) — the element-to-observable projection is null, commonly when the onError/onCompleted handlers are supplied but the element handler was omitted by mistake.

Common situations: Confusion with Subscribe-style handlers where passing null for onNext is legal; copying the Subscribe(source, null, onError, onCompleted) pattern into SelectMany; conditional construction where only error handling was implemented so far.

Related errors

• Value cannot be null. (Parameter 'onError')
• Value cannot be null. (Parameter 'onCompleted')
• Value cannot be null. (Parameter 'func')
• Value cannot be null. (Parameter 'resultSelector')
• new ArgumentNullException(nameof(comparer))

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/14fbd4b488bc3c18. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.StandardSequenceOperators.cs:1380

        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TResult">The type of the elements in the projected inner sequences and the elements in the merged result sequence.</typeparam>
        /// <param name="source">An observable sequence of notifications to project.</param>
        /// <param name="onNext">A transform function to apply to each element.</param>
        /// <param name="onError">A transform function to apply when an error occurs in the source sequence.</param>
        /// <param name="onCompleted">A transform function to apply when the end of the source sequence is reached.</param>
        /// <returns>An observable sequence whose elements are the result of invoking the one-to-many transform function corresponding to each notification in the input sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="onNext"/> or <paramref name="onError"/> or <paramref name="onCompleted"/> is null.</exception>
        public static IObservable<TResult> SelectMany<TSource, TResult>(this IObservable<TSource> source, Func<TSource, IObservable<TResult>> onNext, Func<Exception, IObservable<TResult>> onError, Func<IObservable<TResult>> onCompleted)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

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

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

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

            return s_impl.SelectMany(source, onNext, onError, onCompleted);
        }

        /// <summary>
        /// Projects each notification of an observable sequence to an observable sequence by incorporating the element's index and merges the resulting observable sequences into one observable sequence.
        /// </summary>

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