dotnet/reactive · error · ArgumentNullException

throw new ArgumentNullException(nameof(selector))

Error message

throw new ArgumentNullException(nameof(selector))

What it means

QbservableEx.Expand(source, selector) requires the expansion function as an Expression<Func<TSource, IObservable<TSource>>> so it can be embedded in the queryable expression tree. A null selector throws ArgumentNullException named 'selector'. Delegates are not acceptable here because remote query providers must inspect the expression.

Solutions

1. Supply an expression lambda, e.g. x => Observable.Return(x), typed as Expression<Func<TSource, IObservable<TSource>>>
2. Skip Expand when no expansion rule exists instead of passing null
3. Fix the expression factory that returned null

Example fix

// before
source.Expand(selector); // selector is null
// after
Expression<Func<TSource, IObservable<TSource>>> selector = x => Observable.Return(x);
source.Expand(selector);

if (selector == null)
    throw new InvalidOperationException("Expand selector expression is required");

bool HasSelector<T>(Expression<Func<T, IObservable<T>>> s) => s is not null;

try
{
    var query = source.Expand(selector);
}
catch (ArgumentNullException ex) when (ex.ParamName == "selector")
{
    logger.LogError(ex, "Expand selector was null");
}

When it happens

Trigger: Calling source.Expand(null) — selector held in a nullable variable, conditionally assigned lambda, or produced by a builder that returned null.

Common situations: Config-driven operators where the expansion rule was not configured; refactoring that removed the lambda but kept the call; passing a Func where an Expression was expected and a failed conversion yielded null.

Related errors

• throw new ArgumentNullException(nameof(iteratorMethod))
• throw new ArgumentNullException(nameof(scheduler))
• throw new ArgumentNullException(nameof(source))
• Expected Qbservable.ToQueryable.
• expression

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/9835a24db91ea93f. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/QbservableEx.Homoicon.cs:1025

#endif

#if !STABLE
        /// <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>
        [Experimental]
        public static IQbservable<TSource> Expand<TSource>(this IQbservable<TSource> source, Expression<Func<TSource, IObservable<TSource>>> selector)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (selector == null)
                throw new ArgumentNullException(nameof(selector));

            return source.Provider.CreateQuery<TSource>(
                Expression.Call(
                    null,
                    new Func<IQbservable<TSource>, Expression<Func<TSource, IObservable<TSource>>>, IQbservable<TSource>>(Expand<TSource>).Method,
                    source.Expression,
                    selector
                )
            );
        }
#endif

#if !STABLE
        /// <summary>
        /// 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>

View on GitHub (pinned to 94b5d5ab91)