dotnet/reactive · error · ArgumentNullException

scheduler

Error message

scheduler

What it means

The scheduler-taking overload of Delay throws ArgumentNullException because the scheduler argument is null. A null IAsyncScheduler cannot schedule the delay timer, so the operator rejects it up front. Callers must pass a scheduler such as TaskPoolAsyncScheduler.Default or a test scheduler.

Solutions

1. Pass a concrete scheduler, e.g. TaskPoolAsyncScheduler.Default, NewThreadAsyncScheduler.Default, or your test scheduler.
2. If the scheduler comes from DI/config, verify the registration actually resolved to an instance.
3. Use the simpler Delay(source, dueTime) overload, which defaults to TaskPoolAsyncScheduler.Default.

Example fix

// before
var delayed = source.Delay(TimeSpan.FromSeconds(1), maybeScheduler); // maybeScheduler is null
// after
var delayed = source.Delay(TimeSpan.FromSeconds(1), maybeScheduler ?? TaskPoolAsyncScheduler.Default);

var scheduler = configuredScheduler ?? TaskPoolAsyncScheduler.Default;
var delayed = source.Delay(TimeSpan.FromSeconds(1), scheduler);

static IAsyncScheduler Resolve(IAsyncScheduler? s) => s ?? TaskPoolAsyncScheduler.Default;

try { var d = source.Delay(due, scheduler); }
catch (ArgumentNullException ex) when (ex.ParamName == "scheduler") { var d = source.Delay(due, TaskPoolAsyncScheduler.Default); }

When it happens

Trigger: Calling AsyncObservable.Delay<TSource>(source, dueTime, null) — the second overload with an IAsyncScheduler parameter — passing null for 'scheduler'.

Common situations: A scheduler obtained from configuration/DI that was not registered; a test helper returning null; forgetting to substitute TaskPoolAsyncScheduler.Default when a custom scheduler is unavailable.

Related errors

• source
• observer
• scheduler
• Value cannot be null. (Parameter 'onErrorAsync')
• Value cannot be null. (Parameter 'onCompletedAsync')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/f8ec1e967b667680. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Delay.cs:40

            return Create(
                source,
                dueTime,
                static async (source, dueTime, observer) =>
                {
                    var (sink, drain) = await AsyncObserver.Delay(observer, dueTime).ConfigureAwait(false);

                    var subscription = await source.SubscribeSafeAsync(sink).ConfigureAwait(false);

                    return StableCompositeAsyncDisposable.Create(subscription, drain);
                });
        }

        public static IAsyncObservable<TSource> Delay<TSource>(this IAsyncObservable<TSource> source, TimeSpan dueTime, IAsyncScheduler scheduler)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return Create(
                source,
                (dueTime, scheduler),
                static async (source, state, observer) =>
                {
                    var (sink, drain) = await AsyncObserver.Delay(observer, state.dueTime, state.scheduler).ConfigureAwait(false);

                    var subscription = await source.SubscribeSafeAsync(sink).ConfigureAwait(false);

                    return StableCompositeAsyncDisposable.Create(subscription, drain);
                });
        }
    }

    public partial class AsyncObserver
    {
        public static Task<(IAsyncObserver<TSource>, IAsyncDisposable)> Delay<TSource>(this IAsyncObserver<TSource> observer, TimeSpan dueTime)

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