dotnet/reactive · error · ArgumentNullException

selector

Error message

selector

What it means

Expand(source, selector, scheduler) requires a non-null selector Func<TSource, IObservable<TSource>> that recursively expands each element. A null selector throws ArgumentNullException synchronously at the call site. The selector is the core of the operator; without it no expansion can be defined.

Solutions

1. Provide a valid selector before calling Expand
2. If expansion is optional, use the source directly (or Observable.Empty chains) instead of a null selector
3. Validate DI/config-provided delegates for null at composition time

Example fix

// before
var expanded = source.Expand(expansionStrategy, Scheduler.Default); // strategy null
// after
var expanded = source.Expand(expansionStrategy ?? (x => Observable.Empty<Node>()), Scheduler.Default);

if (selector == null) throw new ArgumentNullException(nameof(selector), "Expand requires a non-null selector.");

bool IsValidSelector<TSource>(Func<TSource, IObservable<TSource>> f) => f is not null;

try { var expanded = source.Expand(selector, scheduler); }
catch (ArgumentNullException ex) when (ex.ParamName == "selector") { log.Error("Expand selector was null", ex); }

When it happens

Trigger: Passing a null selector lambda — commonly an optional strategy delegate that was never assigned, or a method group built via reflection that returned null.

Common situations: Pluggable expansion strategies in tests left null, config-selected delegates that fail to resolve, refactoring that removed the selector body but kept the null variable.

Related errors

• iteratorMethod
• source
• Value cannot be null. (Parameter 'selector')
• Value cannot be null. (Parameter 'conversion')
• Value cannot be null. (Parameter 'source8')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/e31671a5be84617d. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/ObservableEx.cs:82

        /// Expands an observable sequence by recursively invoking selector, using the specified scheduler to enumerate the queue of obtained sequences.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence and each of the recursively expanded sources obtained by running the selector function.</typeparam>
        /// <param name="source">Source sequence with the initial elements.</param>
        /// <param name="selector">Selector function to invoke for each produced element, resulting in another sequence to which the selector will be invoked recursively again.</param>
        /// <param name="scheduler">Scheduler on which to perform the expansion by enumerating the internal queue of obtained sequences.</param>
        /// <returns>An observable sequence containing all the elements produced by the recursive expansion.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="selector"/> or <paramref name="scheduler"/> is null.</exception>
        [Experimental]
        public static IObservable<TSource> Expand<TSource>(this IObservable<TSource> source, Func<TSource, IObservable<TSource>> selector, IScheduler scheduler)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

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

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

            return s_impl.Expand(source, selector, scheduler);
        }

        /// <summary>
        /// Expands an observable sequence by recursively invoking selector.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence and each of the recursively expanded sources obtained by running the selector function.</typeparam>
        /// <param name="source">Source sequence with the initial elements.</param>
        /// <param name="selector">Selector function to invoke for each produced element, resulting in another sequence to which the selector will be invoked recursively again.</param>
        /// <returns>An observable sequence containing all the elements produced by the recursive expansion.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="selector"/> is null.</exception>

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