dotnet/reactive · error · ArgumentNullException

ArgumentNullException: subscription

Error message

ArgumentNullException: subscription

What it means

Thrown when the subscription IAsyncDisposable is null in the capacity overload of GroupByUntil. The subscription disposable links the grouping operator to the outer subscription lifetime and must be provided. The guard fires synchronously with the parameter name 'subscription'.

Solutions

1. Forward the subscription disposable from your composition context
2. Use a no-op stub disposable in tests instead of null
3. Use the high-level extension overloads which obtain the subscription automatically

Example fix

// before
return GroupByUntil(observer, null, ks, ds, 16);
// after
return GroupByUntil(observer, subscription, ks, ds, 16);

if (subscription == null) throw new ArgumentNullException(nameof(subscription));

bool HasSubscription(IAsyncDisposable d) => d is not null;

try
{
    var result = await GroupByUntil(observer, subscription, ks, ds, capacity);
}
catch (ArgumentNullException ex) when (ex.ParamName == "subscription")
{
    // wire the subscription disposable
}

When it happens

Trigger: Calling GroupByUntil(observer, null, keySelector, durationSelector, capacity) when composing operators manually outside the AsyncObservable.Run pipeline.

Common situations: Custom operator scaffolding that never created the disposable; test rigs constructing core overloads by hand.

Related errors

• ArgumentNullException: keySelector
• Value cannot be null. (Parameter 'subscribeAsync')
• Value cannot be null. (Parameter 'observer')
• Value cannot be null. (Parameter 'observer')
• Value cannot be null. (Parameter 'source')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/bf37f883f6ac4a92. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/GroupByUntil.cs:354

                throw new ArgumentNullException(nameof(observer));
            if (subscription == null)
                throw new ArgumentNullException(nameof(subscription));
            if (keySelector == null)
                throw new ArgumentNullException(nameof(keySelector));
            if (durationSelector == null)
                throw new ArgumentNullException(nameof(durationSelector));
            if (comparer == null)
                throw new ArgumentNullException(nameof(comparer));

            return GroupByUntil(observer, subscription, keySelector, durationSelector, int.MaxValue, comparer);
        }

        public static ValueTask<(IAsyncObserver<TSource>, IAsyncDisposable)> GroupByUntil<TSource, TKey, TDuration>(IAsyncObserver<IGroupedAsyncObservable<TKey, TSource>> observer, IAsyncDisposable subscription, Func<TSource, TKey> keySelector, Func<IGroupedAsyncObservable<TKey, TSource>, IAsyncObservable<TDuration>> durationSelector, int capacity)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));
            if (subscription == null)
                throw new ArgumentNullException(nameof(subscription));
            if (keySelector == null)
                throw new ArgumentNullException(nameof(keySelector));
            if (durationSelector == null)
                throw new ArgumentNullException(nameof(durationSelector));
            if (capacity < 0)
                throw new ArgumentOutOfRangeException(nameof(capacity));

            return GroupByUntil(observer, subscription, keySelector, durationSelector, capacity, EqualityComparer<TKey>.Default);
        }

        public static ValueTask<(IAsyncObserver<TSource>, IAsyncDisposable)> GroupByUntil<TSource, TKey, TDuration>(IAsyncObserver<IGroupedAsyncObservable<TKey, TSource>> observer, IAsyncDisposable subscription, Func<TSource, TKey> keySelector, Func<IGroupedAsyncObservable<TKey, TSource>, IAsyncObservable<TDuration>> durationSelector, int capacity, IEqualityComparer<TKey> comparer)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));
            if (subscription == null)
                throw new ArgumentNullException(nameof(subscription));
            if (keySelector == null)
                throw new ArgumentNullException(nameof(keySelector));

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