dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'bufferBoundaries')

Error message

Value cannot be null. (Parameter 'bufferBoundaries')

What it means

Genuine ArgumentNullException: the boundary overload Buffer(source, bufferBoundaries) requires a non-null boundary observable; it is used to subscribe to buffer-closing signals, so null is rejected up front.

Solutions

1. Create a real boundary observable (e.g. from a timer or interval) before calling
2. Fall back to AsyncObservable.Never<TBoundary>() when no boundaries are needed
3. Guard the call site and use a time/count overload instead when boundaries are absent
4. Fix the boundary factory returning null

Example fix

// before
var buf = src.Buffer(boundaries);
// after
var buf = src.Buffer(boundaries ?? AsyncObservable.Never<Unit>());

if (bufferBoundaries == null) bufferBoundaries = AsyncObservable.Never<TBufferBoundary>();

static IAsyncObservable<B> NonNullBoundaries<B>(IAsyncObservable<B> b) => b ?? AsyncObservable.Never<B>();

try { var buf = source.Buffer(bufferBoundaries); }
catch (ArgumentNullException ex) when (ex.ParamName == "bufferBoundaries")
{
    // no boundaries available: fall back to time-based buffering
    var buf = source.Buffer(TimeSpan.FromSeconds(10), 100);
}

When it happens

Trigger: Calling Buffer(source, null); a boundaries factory method returning null; conditional logic that only sometimes creates the boundary stream.

Common situations: Boundary stream created from a timer/event emitter that failed to start; null returned from a lookup of an existing stream by key.

Related errors

• Value cannot be null. (Parameter 'bufferClosingSelector')
• scheduler (Value cannot be null)
• action (Value cannot be null)
• Value cannot be null. (Parameter 'disposable')
• Value cannot be null. (Parameter 'disposable')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/be6db688c5caee6e. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Buffer.cs:182

            return CreateAsyncObservable<IList<TSource>>.From(
                source,
                (timeSpan, count, scheduler),
                static async (source, state, observer) =>
                {
                    var (sink, timer) = await AsyncObserver.Buffer(observer, state.timeSpan, state.count, state.scheduler).ConfigureAwait(false);

                    var subscription = await source.SubscribeSafeAsync(sink).ConfigureAwait(false);

                    return StableCompositeAsyncDisposable.Create(subscription, timer);
                });
        }

        public static IAsyncObservable<IList<TSource>> Buffer<TSource, TBufferBoundary>(this IAsyncObservable<TSource> source, IAsyncObservable<TBufferBoundary> bufferBoundaries)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (bufferBoundaries == null)
                throw new ArgumentNullException(nameof(bufferBoundaries));

            return CreateAsyncObservable<IList<TSource>>.From(
                source,
                bufferBoundaries,
                static async (source, bufferBoundaries, observer) =>
                {
                    var (sourceObserver, boundariesObserver) = AsyncObserver.Buffer<TSource, TBufferBoundary>(observer);

                    var sourceSubscription = await source.SubscribeSafeAsync(sourceObserver).ConfigureAwait(false);
                    var boundariesSubscription = await bufferBoundaries.SubscribeSafeAsync(boundariesObserver).ConfigureAwait(false);

                    return StableCompositeAsyncDisposable.Create(sourceSubscription, boundariesSubscription);
                });
        }

        // REVIEW: This overload is inherited from Rx but arguably a bit esoteric as it doesn't provide context to the closing selector.

        public static IAsyncObservable<IList<TSource>> Buffer<TSource, TBufferClosing>(this IAsyncObservable<TSource> source, Func<IAsyncObservable<TBufferClosing>> bufferClosingSelector)

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