dotnet/reactive · error · ArgumentNullException

source

Error message

source

What it means

ArgumentNullException guard in AsyncObservable.SubscribeOn: the source async observable to be subscribed on the given scheduler was null. Since there is no sequence to schedule subscription work for, the operator throws before touching the scheduler.

Solutions

1. Ensure the observable you call SubscribeOn on is non-null; check the upstream factory/method return value.
2. Guard with a fallback observable if the source may legitimately be missing.
3. If the pipeline is optional, skip SubscribeOn entirely when source is null.

Example fix

// before
var obs = factory.Create(); // may return null
var scheduled = obs.SubscribeOn(scheduler);
// after
var obs = factory.Create() ?? AsyncObservable.Empty<int>();
var scheduled = obs.SubscribeOn(scheduler);

if (source == null)
    throw new InvalidOperationException("Source observable must be non-null before SubscribeOn");

static bool IsObservable<T>(IAsyncObservable<T>? o) => o is not null;

try
{
    var scheduled = source.SubscribeOn(scheduler);
}
catch (ArgumentNullException ex) when (ex.ParamName == "source")
{
    logger.LogError(ex, "Null observable passed to SubscribeOn");
    scheduled = AsyncObservable.Empty<TSource>();
}

When it happens

Trigger: Calling someObservable.SubscribeOn(scheduler) where the receiver observable is null (e.g. a chain built from a null-returning factory or variable), or calling it as a static-style call SubscribeOn(null, scheduler).

Common situations: An observable-producing method returns null on an error path; conditional pipeline construction leaves the source unset; querying an optional configuration for the source.

Related errors

• Value cannot be null. (Parameter 'source')
• Value cannot be null. (Parameter 'source')
• Value cannot be null. (Parameter 'disposable1')
• Value cannot be null. (Parameter 'disposable2')
• Value cannot be null. (Parameter 'onNextAsync')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/de78d0aea1ba4ec6. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/SubscribeOn.cs:15

// Licensed to the .NET Foundation under one or more agreements.
// 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.Reactive.Disposables;

namespace System.Reactive.Linq
{
    public partial class AsyncObservable
    {
        public static IAsyncObservable<TSource> SubscribeOn<TSource>(this IAsyncObservable<TSource> source, IAsyncScheduler scheduler)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return SubscribeOn(source, scheduler, scheduler);
        }

        public static IAsyncObservable<TSource> SubscribeOn<TSource>(this IAsyncObservable<TSource> source, IAsyncScheduler subscribeScheduler, IAsyncScheduler disposeScheduler)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (subscribeScheduler == null)
                throw new ArgumentNullException(nameof(subscribeScheduler));
            if (disposeScheduler == null)
                throw new ArgumentNullException(nameof(disposeScheduler));

            return CreateAsyncObservable<TSource>.From(
                source,
                (subscribeScheduler, disposeScheduler),

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