dotnet/reactive · error · ArgumentNullException

new ArgumentNullException(nameof(resultSelector))

Error message

new ArgumentNullException(nameof(resultSelector))

What it means

The fully-async Generate overload throws ArgumentNullException when the resultSelector delegate is null. resultSelector maps each state to the emitted TResult; without it the operator cannot produce output, so the null check runs before the work is scheduled on the IAsyncScheduler.

Solutions

1. Provide a real resultSelector, e.g. s => new ValueTask<TResult>(Project(s)).
2. If projection is trivial, pass an identity-style lambda such as s => new ValueTask<TState>(s) when TState == TResult.
3. Assert/validate the delegate at composition time with ArgumentNullException.ThrowIfNull(resultSelector) in your own factory before delegating to Generate.

Example fix

// before
Generate(observer, 0, cond, iter, null, timeSel, sched); // throws
// after
Generate(observer, 0, cond, iter, s => new ValueTask<string>("v" + s), timeSel, sched);

if (resultSelector == null) throw new ArgumentNullException(nameof(resultSelector));

static bool HasSelector<TState, TResult>(Func<TState, ValueTask<TResult>> sel) => sel is not null;

try { await Generate(observer, state, cond, iter, sel, timeSel, sched); } catch (ArgumentNullException ex) when (ex.ParamName == "resultSelector") { /* supply fallback selector */ }

When it happens

Trigger: Calling Generate(observer, initialState, condition, iterate, resultSelector, timeSelector, scheduler) with ValueTask delegates (Generate.cs:345) and passing null for Func<TState, ValueTask<TResult>> resultSelector.

Common situations: Wiring selectors from a mapping table or DI container where the entry for resultSelector is missing; copy-pasting an overload call and forgetting to supply the result selector; conditional delegate assignment that evaluates to null.

Related errors

• new ArgumentNullException(nameof(iterate))
• new ArgumentNullException(nameof(timeSelector))
• new ArgumentNullException(nameof(scheduler))
• Value cannot be null. (Parameter 'disposable1')
• Value cannot be null. (Parameter 'disposable2')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/2d5c413de1a3e13b. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Generate.cs:354

                throw new ArgumentNullException(nameof(iterate));
            if (resultSelector == null)
                throw new ArgumentNullException(nameof(resultSelector));
            if (timeSelector == null)
                throw new ArgumentNullException(nameof(timeSelector));

            return Generate(observer, initialState, condition, iterate, resultSelector, timeSelector, TaskPoolAsyncScheduler.Default);
        }

        public static ValueTask<IAsyncDisposable> Generate<TState, TResult>(IAsyncObserver<TResult> observer, TState initialState, Func<TState, ValueTask<bool>> condition, Func<TState, ValueTask<TState>> iterate, Func<TState, ValueTask<TResult>> resultSelector, Func<TState, ValueTask<TimeSpan>> timeSelector, IAsyncScheduler scheduler)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));
            if (condition == null)
                throw new ArgumentNullException(nameof(condition));
            if (iterate == null)
                throw new ArgumentNullException(nameof(iterate));
            if (resultSelector == null)
                throw new ArgumentNullException(nameof(resultSelector));
            if (timeSelector == null)
                throw new ArgumentNullException(nameof(timeSelector));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return scheduler.ScheduleAsync(async ct =>
            {
                var first = true;
                var state = initialState;

                while (!ct.IsCancellationRequested)
                {
                    var hasResult = false;
                    var result = default(TResult);
                    var nextDue = default(TimeSpan);

                    try
                    {

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