What causes "disposeScheduler" in dotnet/reactive?

Use the single-scheduler overload SubscribeOn(source, scheduler) if both phases should use one scheduler.

dotnet/reactive · error · ArgumentNullException

disposeScheduler

Error message

disposeScheduler

What it means

SubscribeOn(source, subscribeScheduler, disposeScheduler) validates the dispose scheduler last. A null disposeScheduler throws ArgumentNullException("disposeScheduler") because resource disposition must be scheduled onto it.

Solutions

Use the single-scheduler overload SubscribeOn(source, scheduler) if both phases should use one scheduler.
Pass a valid dispose scheduler instance explicitly.
Fix whatever returns/assigns null to the dispose scheduler.

Example fix

// before
var obs = source.SubscribeOn(subscribeScheduler, null);
// after
var obs = source.SubscribeOn(subscribeScheduler); // same scheduler for subscribe and dispose

Defensive patterns

Strategy: validation

Validate before calling

if (disposeScheduler == null)
    disposeScheduler = subscribeScheduler; // or AsyncScheduler.DefaultScheduler

Type guard

bool HasDisposeScheduler(IAsyncScheduler? d) => d is not null;

Try / catch

try
{
    var obs = source.SubscribeOn(subscribeScheduler, disposeScheduler);
}
catch (ArgumentNullException ex) when (ex.ParamName == "disposeScheduler")
{
    logger.LogError(ex, "Null dispose scheduler");
    obs = source.SubscribeOn(subscribeScheduler);
}

Prevention

If no separate dispose scheduler is needed, use SubscribeOn(source, scheduler).
Initialize dispose scheduler fields together with subscribe scheduler fields.
Document config keys for both schedulers and fail fast at startup if missing.

When it happens

Trigger: Calling source.SubscribeOn(subscribeScheduler, null) — often when the caller believes disposal reuses the subscribe scheduler, or the dispose scheduler variable was never assigned.

Common situations: Simplifying a call by dropping an argument; config lacking a separate dispose scheduler; refactoring from the single-scheduler overload leaving the second parameter null.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/b3a16dd24790651b. Report an issue: GitHub.

Appendix: source

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

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

            return SubscribeOn(source, scheduler, scheduler);
        }

        public static IAsyncObservable<TSource> SubscribeOn<TSource>(this IAsyncObservable<TSource> source, IAsyncScheduler subscribeScheduler, IAsyncScheduler disposeScheduler)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (subscribeScheduler == null)
                throw new ArgumentNullException(nameof(subscribeScheduler));
            if (disposeScheduler == null)
                throw new ArgumentNullException(nameof(disposeScheduler));

            return CreateAsyncObservable<TSource>.From(
                source,
                (subscribeScheduler, disposeScheduler),
                static async (source, state, observer) =>
                {
                    var m = new SingleAssignmentAsyncDisposable();
                    var d = new SerialAsyncDisposable();

                    await d.AssignAsync(m).ConfigureAwait(false);

                    var scheduled = await state.subscribeScheduler.ScheduleAsync(async ct =>
                    {
                        var subscription = await source.SubscribeSafeAsync(observer).RendezVous(state.subscribeScheduler, ct);

                        var scheduledDispose = AsyncDisposable.Create(async () =>
                        {
                            await state.disposeScheduler.ScheduleAsync(async _ =>

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