dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'windowBoundaries')

Error message

Value cannot be null. (Parameter 'windowBoundaries')

What it means

The Window(source, windowBoundaries) overload throws ArgumentNullException when the windowBoundaries observable is null (runtime message names parameter 'windowBoundaries'). Window openings are driven by emissions from the boundary observable, so null is invalid and rejected before the operator is created.

Solutions

1. Create a real boundary observable (e.g. timer-based) before calling Window.
2. Substitute AsyncObservable.Empty<TWindowBoundary>() when boundaries are intentionally absent.
3. Fix the factory producing windowBoundaries so it never returns null.
4. Guard: if (windowBoundaries == null) throw with a clear message at pipeline setup.

Example fix

// before
var windows = events.Window(boundaries); // boundaries == null
// after
var boundaries = boundariesFactory ?? AsyncObservable.Empty<long>();
var windows = events.Window(boundaries);

if (windowBoundaries is null) throw new ArgumentNullException(nameof(windowBoundaries));
var windows = source.Window(windowBoundaries);

static bool HasBoundaries<TSource, B>(IAsyncObservable<TSource> source, IAsyncObservable<B>? boundaries) => boundaries is not null;

try { var windows = source.Window(boundaries); }
catch (ArgumentNullException ex) when (ex.ParamName == "windowBoundaries") { /* fall back to time-based Window overload */ }

When it happens

Trigger: Calling source.Window(windowBoundaries) with a null boundary observable — e.g. a boundaries factory returning null, or a variable assigned conditionally.

Common situations: A 'tick source' or timer pipeline that failed to initialize; mocking a boundary observable and returning null; configuration that disables the boundary stream without substituting an empty one.

Related errors

• Value cannot be null. (Parameter 'source')
• Value cannot be null. (Parameter 'windowClosingSelector')
• Value cannot be null. (Parameter 'observer')
• Value cannot be null. (Parameter 'subscription')
• nameof(finallyAction)

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/01593fe5dc8d0f02. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Window.cs:141

            if (timeSpan < TimeSpan.Zero)
                throw new ArgumentOutOfRangeException(nameof(timeSpan));
            if (count <= 0)
                throw new ArgumentOutOfRangeException(nameof(count));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return CreateAsyncObservable<IAsyncObservable<TSource>>.From(
                source,
                (timeSpan, count, scheduler),
                static (source, state, observer) => WindowAsyncCore(source, observer, (o, d) => AsyncObserver.Window(o, d, state.timeSpan, state.count, state.scheduler)));
        }

        public static IAsyncObservable<IAsyncObservable<TSource>> Window<TSource, TWindowBoundary>(this IAsyncObservable<TSource> source, IAsyncObservable<TWindowBoundary> windowBoundaries)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (windowBoundaries == null)
                throw new ArgumentNullException(nameof(windowBoundaries));

            return CreateAsyncObservable<IAsyncObservable<TSource>>.From(
                source,
                windowBoundaries,
                static async (source, windowBoundaries, observer) =>
                {
                    var d = new CompositeAsyncDisposable();

                    var (sourceObserver, boundariesObserver, subscription) = await AsyncObserver.Window<TSource, TWindowBoundary>(observer, d).ConfigureAwait(false);

                    var sourceSubscription = await source.SubscribeSafeAsync(sourceObserver).ConfigureAwait(false);
                    await d.AddAsync(sourceSubscription).ConfigureAwait(false);

                    var boundariesSubscription = await windowBoundaries.SubscribeSafeAsync(boundariesObserver).ConfigureAwait(false);
                    await d.AddAsync(boundariesSubscription).ConfigureAwait(false);

                    return subscription;
                });

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