dotnet/reactive · error · ArgumentNullException

throw new ArgumentNullException(nameof(scheduler));

Error message

throw new ArgumentNullException(nameof(scheduler));

What it means

AsyncObservable.Return(value, scheduler) emits a single value on the supplied IAsyncScheduler. Without a scheduler the value cannot be scheduled for emission, so a null scheduler triggers an immediate ArgumentNullException. This lets the developer find the bug at the calling line rather than deep in scheduling code.

Solutions

1. Pass a scheduler instance such as ImmediateAsyncScheduler.Instance, or use the single-argument Return(value) overload when no scheduling is needed.
2. Fix the scheduler factory/DI registration that returns null.
3. Default null schedulers: scheduler ?? ImmediateAsyncScheduler.Instance.

Example fix

// before
AsyncObservable.Return(42, testScheduler); // testScheduler null
// after
AsyncObservable.Return(42, testScheduler ?? ImmediateAsyncScheduler.Instance);

if (scheduler == null) scheduler = ImmediateAsyncScheduler.Instance;
var obs = AsyncObservable.Return(value, scheduler);

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

try { var obs = AsyncObservable.Return(value, scheduler); }
catch (ArgumentNullException ex) when (ex.ParamName == "scheduler") { /* fall back to ImmediateAsyncScheduler.Instance */ }

When it happens

Trigger: Calling AsyncObservable.Return<TSource>(value, null) — the scheduler overload invoked with a null IAsyncScheduler.

Common situations: A test scheduler variable not initialized; DI container missing an IAsyncScheduler registration; switching from the parameterless Return(value) (which uses ImmediateAsyncScheduler.Instance implicitly) to the overload and passing a null 'optional' scheduler.

Related errors

• throw new ArgumentNullException(nameof(observer));
• nameof(scheduler)
• Argument cannot be null (Parameter name: scheduler)
• new ArgumentNullException(nameof(timeSelector))
• new ArgumentNullException(nameof(scheduler))

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/ef06f63c74d38f3e. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Return.cs:20

// The .NET Foundation licenses this file to you under the MIT License.
// See the LICENSE file in the project root for more information. 

using System.Reactive.Concurrency;
using System.Threading.Tasks;

namespace System.Reactive.Linq
{
    public partial class AsyncObservable
    {
        public static IAsyncObservable<TSource> Return<TSource>(TSource value)
        {
            return Create<TSource>(observer => AsyncObserver.Return(observer, value));
        }

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

            return Create<TSource>(observer => AsyncObserver.Return(observer, value, scheduler));
        }
    }

    public partial class AsyncObserver
    {
        public static ValueTask<IAsyncDisposable> Return<TSource>(IAsyncObserver<TSource> observer, TSource value) => Return(observer, value, ImmediateAsyncScheduler.Instance);

        public static ValueTask<IAsyncDisposable> Return<TSource>(IAsyncObserver<TSource> observer, TSource value, IAsyncScheduler scheduler)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return scheduler.ScheduleAsync(async ct =>
            {

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