dotnet/reactive · error · ArgumentNullException

leftDurationSelector

Error message

leftDurationSelector

What it means

GroupJoin needs leftDurationSelector to compute, for each left item, the observable whose completion ends that item's join window. A null selector makes windowing impossible, so ArgumentNullException is thrown at call time (third guard in the method).

Solutions

1. Provide a duration observable per left item, e.g. `l => timer` or `l => AsyncObservable.Empty<TLeftDuration>()` for windows that end on the next right event per your design.
2. If you don't need windowed semantics, use a different operator (e.g. Zip or CombineLatest).
3. Check that your lambda actually binds to leftDurationSelector (first selector slot).

Example fix

// before
var result = left.GroupJoin(right, null, r => r.Take(1), (l, rs) => rs);
// after
var result = left.GroupJoin(right, l => leftTimer, r => r.Take(1), (l, rs) => rs);

if (leftDurationSelector is null) throw new ArgumentNullException(nameof(leftDurationSelector));

bool IsValidSelector<TLeft,TDur>(Func<TLeft, IAsyncObservable<TDur>>? f) => f is not null;

try
{
    var result = left.GroupJoin(right, lds, rds, rs);
}
catch (ArgumentNullException ex) when (ex.ParamName == "leftDurationSelector")
{
    // fall back to a non-windowed operator (Zip/CombineLatest)
}

When it happens

Trigger: Calling GroupJoin with null as the `leftDurationSelector` (Func<TLeft, IAsyncObservable<TLeftDuration>>), e.g. `left.GroupJoin(right, null, rds, rs)`.

Common situations: Migrating from Join/SelectMany-style code that lacked duration semantics; overloads confused after adding duration selectors in a refactor; dynamic query builders that skip optional delegates.

Related errors

• rightDurationSelector
• Value cannot be null. (Parameter 'left')
• Value cannot be null. (Parameter 'right')
• Argument cannot be null (Parameter name: functionAsync)
• Argument cannot be null (Parameter name: observer)

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/54417c62223d8e70. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/GroupJoin.cs:22

using System.Collections.Generic;
using System.Reactive.Disposables;
using System.Reactive.Subjects;
using System.Threading;
using System.Threading.Tasks;

namespace System.Reactive.Linq
{
    public partial class AsyncObservable
    {
        public static IAsyncObservable<TResult> GroupJoin<TLeft, TRight, TLeftDuration, TRightDuration, TResult>(this IAsyncObservable<TLeft> left, IAsyncObservable<TRight> right, Func<TLeft, IAsyncObservable<TLeftDuration>> leftDurationSelector, Func<TRight, IAsyncObservable<TRightDuration>> rightDurationSelector, Func<TLeft, IAsyncObservable<TRight>, TResult> resultSelector)
        {
            if (left == null)
                throw new ArgumentNullException(nameof(left));
            if (right == null)
                throw new ArgumentNullException(nameof(right));
            if (leftDurationSelector == null)
                throw new ArgumentNullException(nameof(leftDurationSelector));
            if (rightDurationSelector == null)
                throw new ArgumentNullException(nameof(rightDurationSelector));
            if (resultSelector == null)
                throw new ArgumentNullException(nameof(resultSelector));

            return Create<TResult>(async observer =>
            {
                var subscriptions = new CompositeAsyncDisposable();

                var (leftObserver, rightObserver, disposable) = AsyncObserver.GroupJoin(observer, subscriptions, leftDurationSelector, rightDurationSelector, resultSelector);

                var leftSubscription = await left.SubscribeSafeAsync(leftObserver).ConfigureAwait(false);
                await subscriptions.AddAsync(leftSubscription).ConfigureAwait(false);

                var rightSubscription = await right.SubscribeSafeAsync(rightObserver).ConfigureAwait(false);
                await subscriptions.AddAsync(rightSubscription).ConfigureAwait(false);

                return disposable;

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