dotnet/reactive · error · ArgumentNullException

Thrown when resultSelector is null (ArgumentNullException…

Error message

Thrown when resultSelector is null (ArgumentNullException, param name: resultSelector)

What it means

For (a foreach-style bind) maps each source element onto a result sequence via resultSelector; null is rejected because the per-element sequence could not be produced. Fix: pass a non-null Func<TSource, IEnumerable<TResult>>.

Solutions

Supply a valid Func<TSource, IEnumerable<TResult>>; use _ => Enumerable.Empty<TResult>() to map to nothing.
Verify the delegate is initialized before query execution.
Add a default at the delegate acquisition point.

Example fix

// before
var result = For(source, projector); // projector may be null
// after
var result = For(source, projector ?? (_ => Enumerable.Empty<TResult>()));

Defensive patterns

Strategy: validation

Validate before calling

if (resultSelector is null) throw new ArgumentNullException(nameof(resultSelector));
For(source, resultSelector ?? (_ => Enumerable.Empty<TResult>()));

Type guard

bool HasSelector<TSrc,TRes>(Func<TSrc,IEnumerable<TRes>> f) => f is not null;

Try / catch

try { var q = For(source, sel); }
catch (ArgumentNullException ex) when (ex.ParamName == "resultSelector") { /* default projection */ }

Prevention

Initialize projection delegates before query construction.
Provide empty-mapping defaults for optional selectors.
Check DI/factory wiring that produces the selector.
Avoid null-conditional delegate passing.

When it happens

Trigger: Calling For(source, null), e.g. the selector obtained from a nullable strategy/injection point.

Common situations: Missing DI registration for the projection delegate; passing obj?.Selector which evaluates to null; a factory that returns null for a fallback case.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/137814f9cfcb8d1c. Report an issue: GitHub.

Appendix: source

Thrown at Ix.NET/Source/System.Interactive/System/Linq/Operators/For.cs:28

    {
        /// <summary>
        /// Generates a sequence by enumerating a source sequence, mapping its elements on result sequences, and concatenating
        /// those sequences.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <typeparam name="TResult">Result sequence element type.</typeparam>
        /// <param name="source">Source sequence.</param>
        /// <param name="resultSelector">Result selector to evaluate for each iteration over the source.</param>
        /// <returns>
        /// Sequence concatenating the inner sequences that result from evaluating the result selector on elements from
        /// the source.
        /// </returns>
        public static IEnumerable<TResult> For<TSource, TResult>(IEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> resultSelector)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (resultSelector == null)
                throw new ArgumentNullException(nameof(resultSelector));

            return source.Select(resultSelector).Concat();
        }
    }
}

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