dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'resultSelector')

Error message

Value cannot be null. (Parameter 'resultSelector')

What it means

System.ArgumentNullException thrown by the seeded overload with result selector when 'resultSelector' (Func<TAccumulate,TResult>) is null, guarded at Aggregate.cs:68. Notably this overload checks resultSelector before func, so a null resultSelector is reported even if func is also null.

Solutions

1. Provide a result selector, e.g. source.Aggregate(seed, acc, l => l.Count).
2. If no projection is needed, switch to the overload without resultSelector.
3. Substitute an identity-like selector (x => x) if the delegate is conditionally null.

Example fix

// before
var result = source.Aggregate(new List<int>(), (acc, x) => { acc.Add(x); return acc; }, maybeSelector); // null

// after
var result = source.Aggregate(new List<int>(), (acc, x) => { acc.Add(x); return acc; }, maybeSelector ?? (l => (IReadOnlyList<int>)l));

if (resultSelector is null) throw new ArgumentNullException(nameof(resultSelector));
var result = source.Aggregate(seed, func, resultSelector);

static bool IsValidSelectorArgs<TAccumulate, TResult>(Func<TAccumulate, TResult>? resultSelector) => resultSelector is not null;

try { var result = source.Aggregate(seed, acc, proj); }
catch (ArgumentNullException ex) when (ex.ParamName == "resultSelector") { /* add identity projection or drop the overload */ }

When it happens

Trigger: Calling source.Aggregate(seed, func, null) — the three-delegate form with a null final projection. Also produced when both func and resultSelector are null (resultSelector check runs first).

Common situations: Projection delegates kept in nullable members not yet initialized; refactoring that removed the result selector but kept the four-argument call; null passed conditionally for optional projection.

Related errors

• Value cannot be null. (Parameter 'func')
• new ArgumentNullException(nameof(comparer))
• ArgumentNullException: comparer
• Value cannot be null. (Parameter 'selector')
• Value cannot be null. (Parameter 'sources')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/4cb6a1b39df47227. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Aggregate.cs:68

        public static IAsyncObservable<TResult> Aggregate<TSource, TResult>(this IAsyncObservable<TSource> source, TResult seed, Func<TResult, TSource, ValueTask<TResult>> func)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (func == null)
                throw new ArgumentNullException(nameof(func));

            return CreateAsyncObservable<TResult>.From(
                source,
                (seed, func),
                static (source, state, observer) => source.SubscribeSafeAsync(AsyncObserver.Aggregate(observer, state.seed, state.func)));
        }

        public static IAsyncObservable<TResult> Aggregate<TSource, TAccumulate, TResult>(this IAsyncObservable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func, Func<TAccumulate, TResult> resultSelector)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (resultSelector == null)
                throw new ArgumentNullException(nameof(resultSelector));
            if (func == null)
                throw new ArgumentNullException(nameof(func));

            return CreateAsyncObservable<TResult>.From(
                source,
                (seed, func, resultSelector),
                static (source, state, observer) => source.SubscribeSafeAsync(AsyncObserver.Aggregate(observer, state.seed, state.func, state.resultSelector)));
        }

        public static IAsyncObservable<TResult> Aggregate<TSource, TAccumulate, TResult>(this IAsyncObservable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, ValueTask<TAccumulate>> func, Func<TAccumulate, ValueTask<TResult>> resultSelector)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (resultSelector == null)
                throw new ArgumentNullException(nameof(resultSelector));
            if (func == null)
                throw new ArgumentNullException(nameof(func));

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