dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'onError')

Error message

Value cannot be null. (Parameter 'onError')

What it means

Thrown by the two-action SubscribeAsync(source, onNext, onError) overload when the onError action is null. The library requires a non-null error handler so faults on the source do not become unhandled exceptions, and throws ArgumentNullException at subscribe time.

Solutions

1. Pass a real error handler such as ex => Log(ex) or ex => { } if truly ignoring.
2. Use the SubscribeAsync(source, onNext) overload, which installs a default error handler that faults the subscription.
3. Ensure injected dependencies used as error handlers are non-null before subscribing.

Example fix

// before
await source.SubscribeAsync(x => Render(x), null);
// after
await source.SubscribeAsync(x => Render(x), ex => Console.Error.WriteLine(ex));

if (onError == null) throw new ArgumentNullException(nameof(onError));
await source.SubscribeAsync(onNext, onError);

bool HasErrorHandler(Action<Exception> onError) => onError is not null;

try
{
    await source.SubscribeAsync(onNext, onError);
}
catch (ArgumentNullException ex) when (ex.ParamName == "onError")
{
    await source.SubscribeAsync(onNext, ex => Console.Error.WriteLine(ex));
}

When it happens

Trigger: Calling SubscribeAsync(source, onNext, null), e.g. omitting error handling because the caller expects exceptions to propagate via the ValueTask await.

Common situations: Migration from callbacks APIs without error handlers, or code where the error logger is injected and can be null in tests.

Related errors

• Value cannot be null. (Parameter 'onCompletedAsync')
• Value cannot be null. (Parameter 'onNext')
• source
• ArgumentNullException(nameof(onError))
• ArgumentNullException(nameof(onCompleted))

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/a98863ff54f8f4ca. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/AsyncObservableExtensions.cs:77

        public static ValueTask<IAsyncDisposable> SubscribeAsync<T>(this IAsyncObservable<T> source, Action<T> onNext)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onNext == null)
                throw new ArgumentNullException(nameof(onNext));

            return source.SubscribeAsync(new AsyncObserver<T>(x => { onNext(x); return default; }, ex => new ValueTask(Task.FromException(ex)), () => default));
        }

        public static ValueTask<IAsyncDisposable> SubscribeAsync<T>(this IAsyncObservable<T> source, Action<T> onNext, Action<Exception> onError)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onNext == null)
                throw new ArgumentNullException(nameof(onNext));
            if (onError == null)
                throw new ArgumentNullException(nameof(onError));

            return source.SubscribeAsync(new AsyncObserver<T>(x => { onNext(x); return default; }, ex => { onError(ex); return default; }, () => default));
        }

        public static ValueTask<IAsyncDisposable> SubscribeAsync<T>(this IAsyncObservable<T> source, Action<T> onNext, Action onCompleted)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onNext == null)
                throw new ArgumentNullException(nameof(onNext));
            if (onCompleted == null)
                throw new ArgumentNullException(nameof(onCompleted));

            return source.SubscribeAsync(new AsyncObserver<T>(x => { onNext(x); return default; }, ex => new ValueTask(Task.FromException(ex)), () => { onCompleted(); return default; }));
        }

        public static ValueTask<IAsyncDisposable> SubscribeAsync<T>(this IAsyncObservable<T> source, Action<T> onNext, Action<Exception> onError, Action onCompleted)
        {

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