dotnet/reactive · error · ArgumentNullException

nameof(scheduler)

Error message

nameof(scheduler)

What it means

Observable.Empty<TResult>(IScheduler) emits the completion notification on the supplied scheduler, which must be non-null. Rx throws ArgumentNullException to ensure the scheduling behavior is explicit; without a scheduler the parameterless overload should be used instead.

Solutions

1. Pass a concrete scheduler such as Scheduler.Default, Scheduler.Immediate, or Scheduler.CurrentThread.
2. Use the parameterless Observable.Empty<TResult>() when no custom scheduling is needed.
3. Null-check the scheduler before calling and substitute Scheduler.Default when null.

Example fix

// before
var obs = Observable.Empty<int>(scheduler); // scheduler null
// after
var obs = Observable.Empty<int>(scheduler ?? Scheduler.Default);

if (scheduler is null) throw new ArgumentNullException(nameof(scheduler));
var obs = Observable.Empty<int>(scheduler);

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

try { var obs = Observable.Empty<int>(scheduler); }
catch (ArgumentNullException ex) when (ex.ParamName == "scheduler") { obs = Observable.Empty<int>(); }

When it happens

Trigger: Calling Observable.Empty<TResult>(scheduler) with a null IScheduler, typically when the scheduler comes from a nullable parameter, property, or service that was not provided.

Common situations: Test code where Scheduler defaults were not injected, DI misconfiguration returning null for IScheduler, or a wrapper API forwarding a null scheduler.

Related errors

• scheduler
• scheduler
• scheduler
• ArgumentNull_Generic
• scheduler

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/db2af8d265ca08b3. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Creation.cs:316

#pragma warning disable IDE0060 // (Remove unused parameter.) Required for type inference
        public static IObservable<TResult> Empty<TResult>(TResult witness)
#pragma warning restore IDE0060
        {
            return s_impl.Empty<TResult>(); // Pure inference - no specialized target method.
        }

        /// <summary>
        /// Returns an empty observable sequence, using the specified scheduler to send out the single OnCompleted message.
        /// </summary>
        /// <typeparam name="TResult">The type used for the <see cref="IObservable{T}"/> type parameter of the resulting sequence.</typeparam>
        /// <param name="scheduler">Scheduler to send the termination call on.</param>
        /// <returns>An observable sequence with no elements.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="scheduler"/> is null.</exception>
        public static IObservable<TResult> Empty<TResult>(IScheduler scheduler)
        {
            if (scheduler == null)
            {
                throw new ArgumentNullException(nameof(scheduler));
            }

            return s_impl.Empty<TResult>(scheduler);
        }

        /// <summary>
        /// Returns an empty observable sequence, using the specified scheduler to send out the single OnCompleted message.
        /// </summary>
        /// <typeparam name="TResult">The type used for the <see cref="IObservable{T}"/> type parameter of the resulting sequence.</typeparam>
        /// <param name="scheduler">Scheduler to send the termination call on.</param>
        /// <param name="witness">Object solely used to infer the type of the <typeparamref name="TResult"/> type parameter. This parameter is typically used when creating a sequence of anonymously typed elements.</param>
        /// <returns>An observable sequence with no elements.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="scheduler"/> is null.</exception>
#pragma warning disable IDE0060 // (Remove unused parameter.) Required for type inference
        public static IObservable<TResult> Empty<TResult>(IScheduler scheduler, TResult witness)
#pragma warning restore IDE0060
        {
            if (scheduler == null)

View on GitHub (pinned to 94b5d5ab91)