dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'iterate')

Error message

Value cannot be null. (Parameter 'iterate')

What it means

The scheduler-based Generate overload throws ArgumentNullException with parameter name 'iterate' when the state-advance function is null. The full .NET message is 'Value cannot be null. (Parameter 'iterate')'.

Solutions

1. Provide a non-null step function like x => x + 1 for the third argument
2. Check argument positions against the (initialState, condition, iterate, resultSelector, scheduler) signature
3. Fail fast at construction of the delegates: if (iterate == null) throw earlier in your own factory

Example fix

// before
AsyncObservable.Generate(0, x => x < 10, null, x => x, Scheduler.Default);
// after
AsyncObservable.Generate(0, x => x < 10, x => x + 1, x => x, Scheduler.Default);

if (iterate == null)
    throw new ArgumentNullException(nameof(iterate));
var obs = AsyncObservable.Generate(state, condition, iterate, resultSelector, scheduler);

static bool HasStep<TState>(Func<TState, TState>? iterate) => iterate != null;

try
{
    var obs = AsyncObservable.Generate(state, condition, iterate, selector, scheduler);
}
catch (ArgumentNullException ex) when (ex.ParamName == "iterate")
{
    logger.LogError("Missing iterate delegate for scheduled Generate");
}

When it happens

Trigger: Calling AsyncObservable.Generate(initialState, condition, null, resultSelector, scheduler) — null step function while specifying a scheduler, e.g. when building arguments programmatically.

Common situations: A null iterate delegate coming from DI or config; five-argument call where an optional lambda was omitted but null was passed explicitly; refactors that reordered parameters.

Related errors

• Value cannot be null. (Parameter 'resultSelector')
• nameof(scheduler)
• scheduler
• scheduler
• ArgumentNull_Generic

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/12e4454fec585aa6. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Generate.cs:29

    {
        public static IAsyncObservable<TResult> Generate<TState, TResult>(TState initialState, Func<TState, bool> condition, Func<TState, TState> iterate, Func<TState, TResult> resultSelector)
        {
            if (condition == null)
                throw new ArgumentNullException(nameof(condition));
            if (iterate == null)
                throw new ArgumentNullException(nameof(iterate));
            if (resultSelector == null)
                throw new ArgumentNullException(nameof(resultSelector));

            return Create<TResult>(observer => AsyncObserver.Generate(observer, initialState, condition, iterate, resultSelector));
        }

        public static IAsyncObservable<TResult> Generate<TState, TResult>(TState initialState, Func<TState, bool> condition, Func<TState, TState> iterate, Func<TState, TResult> resultSelector, IAsyncScheduler scheduler)
        {
            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 (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return Create<TResult>(observer => AsyncObserver.Generate(observer, initialState, condition, iterate, resultSelector, scheduler));
        }

        public static IAsyncObservable<TResult> Generate<TState, TResult>(TState initialState, Func<TState, ValueTask<bool>> condition, Func<TState, ValueTask<TState>> iterate, Func<TState, ValueTask<TResult>> resultSelector)
        {
            if (condition == null)
                throw new ArgumentNullException(nameof(condition));
            if (iterate == null)
                throw new ArgumentNullException(nameof(iterate));
            if (resultSelector == null)
                throw new ArgumentNullException(nameof(resultSelector));

            return Create<TResult>(observer => AsyncObserver.Generate(observer, initialState, condition, iterate, resultSelector));

View on GitHub (pinned to 94b5d5ab91)