What causes "throw new ArgumentNullException(nameof(scheduler))" in dotnet/reactive?

Pass a concrete scheduler such as Scheduler.Default, Scheduler.CurrentThread, or Scheduler.Immediate

dotnet/reactive · error · ArgumentNullException

throw new ArgumentNullException(nameof(scheduler))

Error message

throw new ArgumentNullException(nameof(scheduler))

What it means

The scheduler overload Expand(source, selector, scheduler) requires an IScheduler to time recursive expansions; a null scheduler throws ArgumentNullException named 'scheduler'. Rx requires an explicit scheduler here because Expand's recursion depends on scheduling semantics that cannot be defaulted inside a queryable expression tree.

Solutions

Pass a concrete scheduler such as Scheduler.Default, Scheduler.CurrentThread, or Scheduler.Immediate
Resolve IScheduler from DI and assert non-null before composing the query
Provide an overload without a scheduler if scheduling is genuinely optional

Example fix

// before
source.Expand(x => Observable.Return(x), scheduler); // scheduler is null
// after
var scheduler = configuredScheduler ?? Scheduler.Default;
source.Expand(x => Observable.Return(x), scheduler);

Defensive patterns

Strategy: validation

Validate before calling

if (scheduler == null)
    throw new InvalidOperationException("Expand requires an IScheduler; use Scheduler.Default");

Type guard

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

Try / catch

try
{
    var query = source.Expand(x => Observable.Return(x), scheduler);
}
catch (ArgumentNullException ex) when (ex.ParamName == "scheduler")
{
    logger.LogError(ex, "Expand scheduler was null");
}

Prevention

Coalesce configured schedulers with Scheduler.Default at pipeline setup
Register IScheduler in DI and validate at startup
Never pass null to mean 'default' — use the no-scheduler overload instead

When it happens

Trigger: Calling source.Expand(selector, null) — scheduler from a null configuration value, a TestScheduler variable not yet created, or an IScheduler dependency that failed to resolve.

Common situations: DI failing to provide IScheduler; config where the scheduler name wasn't mapped; passing null deliberately in unit tests to mean 'default' instead of using Scheduler.Default.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/958307dea7f092b8. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/QbservableEx.Homoicon.cs:1057

        /// <summary>
        /// Expands an observable sequence by recursively invoking selector, using the specified scheduler to enumerate the queue of obtained sequences.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence and each of the recursively expanded sources obtained by running the selector function.</typeparam>
        /// <param name="source">Source sequence with the initial elements.</param>
        /// <param name="selector">Selector function to invoke for each produced element, resulting in another sequence to which the selector will be invoked recursively again.</param>
        /// <param name="scheduler">Scheduler on which to perform the expansion by enumerating the internal queue of obtained sequences.</param>
        /// <returns>An observable sequence containing all the elements produced by the recursive expansion.</returns>
        /// <exception cref="ArgumentNullException">
        /// <paramref name="source" /> or <paramref name="selector" /> or <paramref name="scheduler" /> is null.</exception>
        [Experimental]
        public static IQbservable<TSource> Expand<TSource>(this IQbservable<TSource> source, Expression<Func<TSource, IObservable<TSource>>> selector, IScheduler scheduler)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (selector == null)
                throw new ArgumentNullException(nameof(selector));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return source.Provider.CreateQuery<TSource>(
                Expression.Call(
                    null,
                    new Func<IQbservable<TSource>, Expression<Func<TSource, IObservable<TSource>>>, IScheduler, IQbservable<TSource>>(Expand<TSource>).Method,
                    source.Expression,
                    selector,
                    Expression.Constant(scheduler, typeof(IScheduler))
                )
            );
        }
#endif

#if !STABLE
        /// <summary>
        /// Runs all specified observable sequences in parallel and collects their last elements.
        /// </summary>
        /// <param name="provider">Query provider used to construct the <see cref="IQbservable{T}"/> data source.</param>

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