dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'scheduler')

Error message

Value cannot be null. (Parameter 'scheduler')

What it means

This ObserveOn overload requires a non-null DispatcherScheduler instance; it throws ArgumentNullException('scheduler') when null. The scheduler's Dispatcher and Priority are read immediately (scheduler.Dispatcher, scheduler.Priority), so a null wrapper cannot be tolerated.

Solutions

1. Create the DispatcherScheduler explicitly from a valid Dispatcher: new DispatcherScheduler(Dispatcher.CurrentDispatcher).
2. Verify the DI container registration provides a DispatcherScheduler and that resolution succeeded before calling ObserveOn.
3. If the scheduler is optional in some hosts, branch: use the DispatcherScheduler overload only when available, otherwise a standard IScheduler overload.

Example fix

// before
DispatcherScheduler sched = _schedulerProvider.Get(); // may return null
var ui = source.ObserveOn(sched);
// after
var sched = _schedulerProvider.Get() ?? new DispatcherScheduler(Application.Current.Dispatcher);
var ui = source.ObserveOn(sched);

if (dispatcherScheduler == null) dispatcherScheduler = new DispatcherScheduler(Application.Current.Dispatcher);

bool schedulerReady = dispatcherScheduler != null && dispatcherScheduler.Dispatcher != null;

try { var ui = source.ObserveOn(dispatcherScheduler); } catch (ArgumentNullException ex) when (ex.ParamName == "scheduler") { var ui = source.ObserveOn(Scheduler.Default); }

When it happens

Trigger: Calling source.ObserveOn(dispatcherScheduler) where the DispatcherScheduler was never created or was injected/looked up as null, e.g., new DispatcherScheduler not yet assigned, or a container returning null for the scheduler registration.

Common situations: DI registration missing for DispatcherScheduler; creating the scheduler lazily on a background thread; accessing a static scheduler singleton before WPF application initialization.

Related errors

• throw new ArgumentNullException(nameof(scheduler));
• action (Value cannot be null)
• scheduler
• scheduler
• scheduler

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/90d18f112c910ece. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive.Wpf/System.Reactive.Linq/DispatcherObservable.cs:86

        /// <summary>
        /// Wraps the source sequence in order to run its observer callbacks on the specified dispatcher scheduler.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Source sequence.</param>
        /// <param name="scheduler">Dispatcher scheduler to notify observers on.</param>
        /// <returns>The source sequence whose observations happen on the specified dispatcher scheduler.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="scheduler"/> is null.</exception>
        public static IObservable<TSource> ObserveOn<TSource>(this IObservable<TSource> source, DispatcherScheduler scheduler)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

            if (scheduler == null)
            {
                throw new ArgumentNullException(nameof(scheduler));
            }

            return ObserveOn_(source, scheduler.Dispatcher, scheduler.Priority);
        }


        /// <summary>
        /// Wraps the source sequence in order to run its observer callbacks on the dispatcher associated with the specified object.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Source sequence.</param>
        /// <param name="dispatcherObject">Object to get the dispatcher from.</param>
        /// <returns>The source sequence whose observations happen on the specified object's dispatcher.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="dispatcherObject"/> is null.</exception>
        public static IObservable<TSource> ObserveOn<TSource>(this IObservable<TSource> source, DispatcherObject dispatcherObject)
        {
            if (source == null)
            {

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