dotnet/reactive · error · ArgumentNullException

ArgumentNullException(nameof(scheduler))

Error message

ArgumentNullException(nameof(scheduler))

What it means

The Merge(IEnumerable<IObservable<TSource>>, int, IScheduler) overload throws ArgumentNullException when the scheduler argument is null. Rx requires an explicit IScheduler for this overload because scheduling governs how concurrent subscriptions are orchestrated.

Solutions

1. Pass a concrete scheduler such as Scheduler.Default, TaskPoolScheduler.Default, or CurrentThreadScheduler.Default.
2. If injection is used, verify the IScheduler registration in the DI container.
3. Coalesce with Scheduler.Default at the call site if a null scheduler is acceptable to treat as default.
4. Check initialization order so the scheduler field is assigned before Merge runs.

Example fix

// before
var merged = sources.Merge(4, _scheduler); // _scheduler is null
// after
var merged = sources.Merge(4, _scheduler ?? Scheduler.Default);

if (scheduler is null) scheduler = Scheduler.Default;

static bool HasScheduler(IScheduler s) => s is not null;

try { var merged = sources.Merge(4, scheduler); }
catch (ArgumentNullException ex) when (ex.ParamName == "scheduler")
{
    var merged = sources.Merge(4, Scheduler.Default);
}

When it happens

Trigger: Calling Merge(sources, maxConcurrent, scheduler) where scheduler is null — commonly a field not yet assigned, a factory that returned null, or passing a dependency that was never injected.

Common situations: DI container misconfiguration where IScheduler was not registered; using a scheduler variable before initialization in tests; migrating code that removed Scheduler.Default.

Related errors

• action (Parameter 'action')
• action (Parameter 'action')
• comparer (Parameter 'comparer')
• scheduler (Parameter 'scheduler')
• scheduler (Parameter 'scheduler')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/aaa88c86cbb420d5. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Multiple.cs:548

        /// <param name="scheduler">Scheduler to run the enumeration of the sequence of sources on.</param>
        /// <returns>The observable sequence that merges the elements of the observable sequences.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="sources"/> or <paramref name="scheduler"/> is null.</exception>
        /// <exception cref="ArgumentOutOfRangeException"><paramref name="maxConcurrent"/> is less than or equal to zero.</exception>
        public static IObservable<TSource> Merge<TSource>(this IEnumerable<IObservable<TSource>> sources, int maxConcurrent, IScheduler scheduler)
        {
            if (sources == null)
            {
                throw new ArgumentNullException(nameof(sources));
            }

            if (maxConcurrent <= 0)
            {
                throw new ArgumentOutOfRangeException(nameof(maxConcurrent));
            }

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

            return s_impl.Merge(sources, maxConcurrent, scheduler);
        }

        /// <summary>
        /// Merges elements from two observable sequences into a single observable sequence.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequences.</typeparam>
        /// <param name="first">First observable sequence.</param>
        /// <param name="second">Second observable sequence.</param>
        /// <returns>The observable sequence that merges the elements of the given sequences.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="first"/> or <paramref name="second"/> is null.</exception>
        public static IObservable<TSource> Merge<TSource>(this IObservable<TSource> first, IObservable<TSource> second)
        {
            if (first == null)
            {
                throw new ArgumentNullException(nameof(first));

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